How to get GetURL variable and pass it to another form?

How to get GetURL variable and pass it to another form?

I have a form that is filtered using a url variable.  Example:

..//#Form:Income?invoice=123

When I click the "make payment" button from mu invoice page. The user will be redirected to another payment page and the payment page will inherit the selected invoice ID (in this case is 123).

How should I code this in my "make payment" page?