i'm attempting to get my website www.windowcleaningcoach.com/zencart to use sage pay form simulator

Apparently i need to place this: 'https://test.sagepay.com/Simulator/VSPFormGateway.asp'

somewhere, maybe in the action part of the form tag??

basically i don't know where the location is that i should place
'https://test.sagepay.com/Simulator/VSPFormGateway.asp'

has anyone any ideas?