There's a thread on the old forums that talks about this; the same topic was discussed more recently as well. The main takeaway is that it's possible to implement a customization to handle this, though there isn't an out-of-the-box way to handle it.

The solution was to modify the WebLink project to take credentials via URL and call Login() on the LoginInterface8 control with those credentials. Brian also suggested passing a cookie to WebLink instead of just putting the credentials in the URL in the case where you have a referring application that is on the same domain as WebLink (to be more secure).

You could also try some custom javascript. For example, http://stackoverflow.com/questions/14070105/pre-fill-form-field-via-url-in-html should allow you to autofill the login information from the URL using custom javascript on the login page.