Steve,
there are several ways how to approach this problem. It always involves inspecting the html source code. Often it is useful to also sniff headers; a cool extension for firefox which does that is
LiveHTTPHeaders.
In general, passing parameters via url only works when you are dealing with GET variables. Have a look
here to understand the fundamental difference between the two available methods, GET and POST.
So when you inspect an html source code, and figure out that the login form (or whatever form your are inspecting) uses something like
method="get", then it is possible to add the transmitted variables to the url.
Another great tool to check what variables are being passed within a form is Firefox's "
View Page Info" (right mouse click). In the Tab Forms, it shows you a form's fields, including name, type and current value.
Tell me if that helps you in any way.