How to send a Request with Form-Parameters as Pre-Request Script?

So basically, for my Restful Webservice I need to log in as an Admin, before I can start an automated series of tests.
In there any way to send a login request (http://localhost:8080/Webservice_war_exploded/resource/user/login) with Form-Parameters username=admin1 & password=1234 as a Pre-Request Script?
Alternatively can I link to a already existing test case that that logs in as an admin?