1) Click on http://help.ea.com/en/contact-us/new/
2) Select the product, " Origin powered by EA" (have to be logged in)
3)Hit Next
4) Select the platform, "PC"
5)Hit next
6) Select the topic, "General Questions"
7) When it says, " Please tell us a little more about your issue" type, "I recieved an xbox one beta code for Titanfall when I signed up for the PC"
8 )Hit Next
9) Hit Live chat
It takes about an hour to get on the livechat with someone, tell them that you got the wrong code and they should give your account access to the beta.
Update: Last 7ish hours of beta, it may be "open beta" but for PC users who didn't sign up it's not and this still works. All you need to do is get live chat open. Titanfall > PC or Origin > Demo/beta should work. Then use same method.