If this field has an edit box format, you can use the OnObjectEnter script trigger to run a script that determines whether the field may or may not be entered.
Thank you .... but this onobjectenter trigger does not work on IWP and I do not know if it works for FMGO. So I will implement the idea for the full FM terminals but is there another way to achieve the same for those who are using IWP or FMGO ?
I believe triggers work in GO, from what I've read. They don't work in response to user actions in IWP. If I had known that detail, I would have suggested a more cumbersome, approach that works without triggers. This method works if the field is set up as a drop down or pop up I'm not completely sure it will work in IWP, you'll have to try it and see. (Otherwise, I think you're stuck with maintaining two layouts.)
Take this field out of the tab order so that it cannot be entered by pressing Tab, enter or Return. Place a transparent button over the top of the field. Now set up this button to run the script I suggested previously.