First, you really should not use the old "formmail.pl" anymore, because it has a history of being insecure and susceptible to abuse by spammers. The nms-formmail script was designed as a direct drop-in replacement for formmail.pl, but one which is better-programmed and more secure; if you're already familiar with formmial.pl, you should find nms-formail quite familiar, but you'll wind up with a superior script. Even the original author of formmail.pl is recommending the nms replacements over his own, older scripts:
I'm not clear what you mean about the "URL page", unless you're asking about the Common Notification URL or Payment Variable Information URL for E-junkie's integration feature? If that's what you meant, then if your script is installed here:
...then that would also be the URL you specify for E-junkie integration.
If you want to provide a Date field or menu for your product's E-junkie purchase button, you could do that by enabling the "Variations which tell more..." setting for the product, then on the following screen you would name Option 1 something like "Date" and either provide a list of acceptable dates for Option 1 below that, or leave the list blank to show buyers a text field to fill in manually.