http://help.ea.com/uk/origin/origin/
Scroll to bottom of page "contact us" add a topic "titanfall beta wrong key".
Then tell operator that you have recieved the wrong key, an they gave you a xbox one key an you need a pc.
They don't ask for proof of key, a load of my facebook friends and my self included got are keys this way.
Here is my chat with the EA op,
PRADEEP-Thanks for contacting EA Customer Experience, my name is Pradeep. How may I assist you today?
you -hi ive had a titanfall code but the wrong one, ive had a xboxone code but need pc
Pradeep -I apologize inconvenience caused, that you have received wrong platform code. PradeepThere is not need to worry now.
Pradeep -I do understand your issue and able to fix it. youthank you
Pradeep -May you please allow me a minute to resolve your issue?
you -yes thats fine
Pradeep -Thank you for waiting. I appreciate your patience.
Pradeep -Please login on Origin application. I have added Beta game on your Games library.
Pradeep -You may check and let me know, if it works for you.
you -Many thanks
Took me 55mins of waiting in a cue last night, took less than 2 mins for Op to add game to my library
An that 55min wait was worth every minute