Pick up user's login, but prompt for password?

I’m trying to get a script that mimics what happens when you manually connect to a sever. Specifically, show a dialog that contains the user’s username for that server (or even blank would be OK) and an empty password field.

The closest I’ve come to this so far is to leave the password blank but fill in generic text for the username, such as:

mount volume “afp:// your_name_here:@URL/volume_name”

I also don’t understand why I can’t specify one field as blank but not both. Either way it should bring up the login dialog, in my opinion.

Well, it’ll accept a single space as the “username” — looks a little more obvious that a real username will need to be inputted, however the cursor is at the password field initially.

I guess I should rephrase my request: I’d like the script to prompt first at the username field, as you’d expect.