From the Launch Screen, select Trainz Settings then the MyTrainz tab. There you will see, under MyTrainz account, a link to your login/registration. I am not sure exactly how it works now since it has changed since I last needed to enter my details, but enter your forum name (Mambo1954) and your forum password. You will/should get a message stating that the details have been accepted. You will then need to exit from the Trainz Settings and then exit from Trainz completely and then restart Trainz.
Development: Trainz Plus - b119450
Testing: Trainz Plus beta - b119450
GTX1050Ti, RTX3060Ti and RTX3050Ti, Win 10 & 11