You can sign up to Amber Black using this page here: https://amber.app/sign-up/
To sign up:
Input your Amber account email
Select your preferred subscription type as Monthly or Annually
Input your payment method
Signing up will begin with a 1-week trial before being charged, so you can try it risk-free. This is limited to 1 trial per user, so if you have completed an Amber Black trial in the past you will be charged immediately.
Please note that if you are already enrolled in Amber Black you will be unable to sign up again.