how to dynamically assign a prompt table depending on a drop down value (see image below)
The table behind the grid is PORTAL_SECDYVW and as you can see from the image below, the PORTAL_AUTHNAME field has %EDITTABLE defined as a prompt table. The PORTAL_AUTHNAME is the "Name" column you see on the grid.

Now, we would want to assign table PSCLASSDEFN_SRC as prompt if the drop down value is Permission list and PSROLEDEFN_SRCH if the value is Role.

/*Record PeopleCode: PORTAL_SECDYVW.PORTAL_AUTHNAME.FieldChange*/
Evaluate PORTAL_SECDYVW.PORTAL_PERMTYPE2
When "P"
   DERIVED.EDITTABLE = "PSCLASSDEFN_SRC";
   PORTAL_SECDYVW.DESCR = PSCLASSDEFN.CLASSDEFNDESC.Value;
   Break;
   
When "R"
   DERIVED.EDITTABLE = "PSROLEDEFN_SRCH";
   PORTAL_SECDYVW.DESCR = PSROLEDEFN.DESCR.Value;
   Break;

When-Other
   
End-Evaluate;