I tried 

CastSpellByID(spellID, "targetName")

sometimes it works but sometimes it is doing strange stuff and target my toon or is casting at my current target.
May someone got a idea how i can realize it lie healbot for example that i don't need a target selected to heal it?

UPDATE:
I found a solution for it but my problem now is that the bot only starts healing if he has a target doesn*t matter what. Any tipps?

Like this (not tested):

    public static void CastByPlayerName(int spellId, string playerName)
    {
        string lua = "CastSpellByID(spellID, 'focus')";
        var target = ObjectManager.GetObjectWoWPlayer().FirstOrDefault(p => p.Name == playerName);
        try
        {
            if (target != null && !target.IsValid)
                return;

            if (target.IsMyTarget)
            {
                Lua.LuaDoString(lua.Replace("spellID", spellId.ToString()).Replace("'focus'", "'target'"));
            }
            else if (target.Guid == ObjectManager.Me.FocusGuid)
            {
                Lua.LuaDoString(lua.Replace("spellID", spellId.ToString()));
            }
            else
            {
                var currentFocus = ObjectManager.Me.FocusGuid;
                ObjectManager.Me.FocusGuid = target.Guid;
                Lua.LuaDoString(lua.Replace("spellID", spellId.ToString()));
                ObjectManager.Me.FocusGuid = currentFocus;
            }

        }
        catch { }
    }

In second argument of castspellby... (in legion version), it is not player name, but it is unitid http://wow.gamepedia.com/UnitId

Thanks for you answer but i'm using the focus for the tank so i'm not able to use your way.

I wrote another method for it wich works fine. The only problem i noticed is that i always need a target to get the bot starting the routine. If i dont have a target the bot does nothing.

Heal method:

        /// <summary>
        /// Cast a heal
        /// </summary>
        /// <param name="spell">The heal you want to cast</param>
        /// <param name="target">The target you want to heal</param>
        /// <param name="healthProcent">The health procent we want to heal</param>
        /// <param name="buffTimeLeft">Recast if buff is under the given time</param>
        /// <param name="stacks">How much stacks you want at the target</param>
        /// <param name="debuff">The debuff we are looking for</param>
        /// <param name="owner">Flag that determines if we need to be the owner</param>
        /// <returns>Returns true if we can cast the spell otherwise false</returns>
        public static bool CastHeal(Spell spell, WoWUnit target, int healthProcent, int buffTimeLeft = 0, int stacks = 0, Spell debuff = null, bool owner = true)
        {
            // Need heal?
            if (target.HealthPercent > healthProcent)
            {
                // Skip
                return false;
            }

            // Wait until global cooldown is done
            Thread.Sleep(SpellManager.GlobalCooldownTimeLeft());

            bool hasDebuff;
            if (debuff != null)
            {
                hasDebuff = Functions.HasBuff(debuff, target, buffTimeLeft, stacks, owner);
            }
            else
            {
                hasDebuff = Functions.HasBuff(spell, target, buffTimeLeft, stacks, owner);
            }

            // Validate spell
            if (!ObjectManager.Me.IsStunned && !ObjectManager.Me.IsDead && !ObjectManager.Me.IsCast && !target.IsDead && spell.KnownSpell && spell.IsSpellUsable && spell.IsDistanceGood && !hasDebuff)
            {
                if (target.Guid == ObjectManager.Me.Guid)
                {
                    // Cast on self
                    Lua.LuaDoString($"CastSpellByID({spell.Id}, \"player\")");
                }
                else
                {
                    // Cast on target
                    Lua.LuaDoString($"CastSpellByID({spell.Id}, \"{target.Name}\")");
                }

                // Log
                Logging.WriteDebug($"Cast: {spell.NameInGame}");

                // Return
                return true;
            }

            // Return
            return false;
        }

HasBuff method:

        /// <summary>
        /// Determines if the given target has the buff with the given conditions
        /// </summary>
        /// <param name="spell">The spell you want to check</param>
        /// <param name="target">The target you want to check</param>
        /// <param name="buffTimeLeft">Recast if buff time is under the given time</param>
        /// <param name="stacks">How much stacks you want at the target</param>
        /// <param name="owner">Flag that determines if we need to be the owner</param>
        /// <returns>Returns true if the target has the spell on it with the given conditions otherwise false</returns>
        public static bool HasBuff(Spell spell, WoWUnit target, int buffTimeLeft = 0, int stacks = 0, bool owner = true)
        {
            // Get target auras
            List<Aura> auraList = target.GetAllBuff();

            // Get aura
            Aura aura = null;
            if (owner)
            {
                // Set
                aura = auraList.Where(s => s.ToString().Contains(spell.Name) && s.Owner == ObjectManager.Me.Guid).FirstOrDefault();
            }
            else
            {
                // Set
                aura = auraList.FirstOrDefault(s => s.ToString().Contains(spell.Name));
            }

            // Any found?
            if (aura != null)
            {
                // Validate
                if (aura.TimeLeftSeconde > buffTimeLeft && aura.Stack >= stacks)
                {
                    // Return
                    return true;
                }
            }

            // Return
            return false;
        }

Hope it will help some people who dealing with stuff like that :)

Info: It wont work with copy & paste, you need to correct some lines, i'm sorry for that but this are just snippets out of my own framework.