During past Alpha tests there was no way to sign up to Alpha trials.
During past Alpha trials, It has been invite only via e:mail and I can only say that as I've been in BF3, BF1 and BFV Alpha tests.
This was the case for the BF3 Alpha, BF4 Alpha, BF1 and BFV Alpha trials also though this may have changed with the playtesting sign up area added to the account settings page.
Just supposing there is an Alpha to come of the next title, There will usually be a non-disclosure agreement ( NDA ) that you have to adhere to when taking part.
There will be certain things you cannot do..
Some of the things covered are :
- No streaming
- No sharing content ( screenshots for example ) outside of specifically designated areas where access is only available to those in the test
- No recording of content
And a few others specific to the NDA at the time.
I'm not sure if the playtesting sign up page from your account page will guarantee access, It would probably help though.