This is currently the only info available, that they will be sent out shortly before the Beta goes live.
Tokens will be sent close to the beginning of your Beta start date. Specific timing TBA.
Activision will send your Early Access Beta Token to the registered email address that you have provided in your Call of Duty account. To ensure you receive updates on the Beta, please double-check your email address and make sure it is up to date in our system. Log in to your Call of Duty profile to check or make changes to your registered email address.
After the Early Access Beta Tokens are emailed, they will also be available at callofduty.com/beta.
It doesn't matter when you pre ordered, everyone will receive an Email with a token shortly before the Beta goes live, regardeless of when they did their pre order.