I have an account that was created before the free trial event and does not have ESO purchased on it. I used this link
https://www.elderscrollsonline.com/en-us/freeplay and signed in with the account but all it does is take me to my account page and says "You have not yet made any game purchases. Please click Redeem Code below to enter your Game Registration Code or Buy Game to purchase the digital version." It does not give any prompt for a trial code and there is nothing in my email about it.