Sending parameters to an existing web application

[Simon]

I need to pre-fill a bunch of textboxes (with info from my current
application) on an existing application on the server. The receiving page
contain dropdownlists and textboxes. Both pages are .aspx, and I do have
access to edit the receiving page's code.

So here's my question: What's a good way to do this? All my googling is
leading me to tips on sending parameters between pages within the same
Project (I think). But these are two different "Projects" (i hope that's
the correct term). I am faily new to this stuff, so I'd highly appreciate
any help. Thanks in advance!