GST group code security

Hi all... I am working in Nav 13, I am trying to put a security on my GST group code.

In this i have  structures named 'ST-38' and 'ST-41'(Purchase header), i would be needing a security in which if i select 'ST-38'(on the header) it shows me only TG-01 & TG-02(on the line) and if i select 'ST-41' it shows me only TG-03 & TG-04(on the line) in my selection drop down.


All kinds of helps would be appreciated.