You can use this HTML code as a example of how to make a pay now button on your website.
<html><form method='POST' action= 'https://swp.paymentsgateway.net/co/default.aspx'><table cellSpacing='0' cellPadding='0' border='0'><tr><td align='right' width='300'></td><td align='left' width='200'><input type='hidden' name='pg_api_login_id' value='REPLACE_WITH_YOUR_OWN'/></td></tr><tr><td align='right' width='300'></td><td align='left' width='200'><INPUT TYPE=SUBMIT value='Pay Now'><br></td></tr></table></form></html>
The following article provides information how to setup Secure Web Pay ("SWP-Checkout"):