Setting trigger that locks one field based on selections in another field

[JChamber]

Hello,

I'm trying to write a trigger that will lock one field based on specific values users select in another field. For example, if the user selects value "A" from Field1, the values for Field2 are locked and users are prevented from making selections on Field2. (And if the user selects any value other than A in Field1, Field2 is not locked and users can make selections on Field2).

Here's what I've tried without success:

Document properties  -->  Field Event Triggers  -->  OnSelect  [for Field1] -->  AddAction -->  "Selection / Lock Field"  -->  Field -->  =if(GetFieldSelections(Field1) = 'A',Field2,)

I've also tried just  =if(Field1 ='A',Field2,) but that also doesn't work.

Any advice or suggestions for how to build that trigger would be greatly appreciated. Thank you.

-John