How to force a mandatory update activity upon entering a particular CM phase

Dear folks,

I'm working on a new workflow that has a "Hold" phase that will allow users to "park" a change ticket if they need to for some reason.  What I want to do is force the user to add an update upon entering that phase explaining the reason why this case is now on "hold".  I figure that using a format control tied to the form used in that particular phase would do the trick, so I did the following:

Validation tab:

 

Update: current.phase in $file="Hold" and current.phase in $file0~="Hold"

Focus: $cm.update (I also tried "1 in $cm.update")

Validation: not (null(1 in $cm.update)) or $cm.update~={""}

Message: Please explain why are you holding this request

No luck so far.  What am I missing here?  Can I use the $cm.update variable that way in a format control?  I'm also considering defining a field used only in that phase and that can be updated every time the workflow moves into this phase (which I assume may happen more than once). 

 

I'll appreciate any suggestion you may provide.

Kind regards

Ulises