Create a Photon project with iMessage / RCS as its provider, then copy its Project ID and Project Secret from Spectrum, Configure into Drift under Messaging, Connect.
In Photon, add yourself as a user with your own mobile number, and copy the number under Texts On into Drift as the Photon number.
Finally, text the 10-digit pairing code Drift shows to that number from your phone. Until then the number answers no one; once paired, it only answers you.