when do i pay the co-pay for the hospital?

Hi i wanted to know, when and where do i pay for my co-pay for the hospital?  I have medicare, live in michigan and going to grand partners. Thanks

I know they requested mine when the hospital called to pre-register me for my pre op tests. They took my credit card over the phone and the receipt was waiting when I went in for the tests (which they then sent me home from because my surgery was pushed out to 9/13).