Hi Jian ,
You can also try screen variant to make it a required field.
Use SHD0 and create a screen variant of the transaction & make the partner function key as mandatory.
Add this scren variant to a transaction variant & activate it.
Creation of Screen Variant with Content in a Standard T Code.
Once the field is made mandatory by variant , then user will not be able to proceed further without entering function key.
Hope it helps !!
B/r
Sachin