OpenText VIM Custom Button in SAP GUI Approval Screen

Hi All

Please help. I'm trying to configure a custom button for SAP GUI Approval Screen. Configuration done in /OPT/OAP_DETL_BUTTON, and I assigned a simple function module to call another t-code for displaying a summary form.

The button is showing up in the approval screen but when I clicked, nothing happen. I try to put an URL, still nothing happens. The button doesn't seem to react to anything. Tried ABAP debug via /h, when the button clicked its not doing anything. Is there any other setup that I missed? Please help.