You will need an existing WP Dev Centre account, and not every application will be accepted:
You must complete this application in order to be eligible. At the end of the application period, we'll evaluate the applications and select participants at our sole discretion. Not everyone who applies will be accepted into the program, and we may discontinue this program or your participation in this program at any time.
Applications will be accepted until close of business on Monday 17th September in Redmond.
You can start the process at Microsoft's Connect site.
Windows Phone Dev Center as it looks today.