Most relevant config points from my video here.

The most important part is setting the "Digest Credentials", that is the password that the phone will use to authenticate.

Device -> Phone

Set a fake MAC address in the right format eg. AAABBBCCCDDD.
Remember to assign the phone to the user you created earlier.

Set the "Third-party SIP Device Basic - Standard SIP" security profile as below.

Scroll down and set the Digest user to the user we sent earlier.

Remember to create a DN, your phone will then authenticate with the DN as the username and the password of the "Digest User".