Ordering a Unit To Click On A Spell Without Casting It

[Pandamanana]

I'm trying to trigger a skill that works like this.
When ordered to cast the ability (frenzy) I want the unit or player to be ordered to BEGIN casting a hidden ability (hidden in a spellbook that is disabled) that is a point-target ability, meaning that when you go to cast Frenzy, your unit stops (keeping him from casting Frenzy) and his icon changes to the point-target icon (he begins to cast the second ability) which I can then use to create the rest of the spell.
[Q&A]
Q1. Why not just use a point-target cast skill instead of all this complicated crap?
A1. I'm making a multi-purpose skill, so clicking to cast it does one function while clicking to set it to auto-cast does another.

 

[Renendaru]

If you set a hotkey to the ability you can use the call 'call ForceUIKey("*Letter in caps*")'.

I'm not sure about the name for the command but it's along the lines of that.

 

[Pandamanana]

I cannot use the Force UI trigger when the ability in question is hidden in a disabled spellbook.

 

[Darthfett]

I cannot use the Force UI trigger when the ability in question is hidden in a disabled spellbook.

The ability cannot be used if it is hidden in a disabled spellbook, period. The only exception is a passive ability, which doesn't technically count, as there is no order being run.

An idea would be to add the ability to the unit, issue the order, and remove the ability.