Populating selection screen of one prog by calling from another prog

This is regarding variant population.

I need to populate values in selection screen fields of one program (program 2)by calling from another executable ABAP program(.Program 1)

I can identify the variant that i need ot populate.But there is a pushbutton in the selection screen of Program 2 which on clicking leads to some more fields in a pop up screen.How can I populate values in those fields there ?Please help.

Thanks & Regards,

Savitha