Welcome Trainz2000000 (what a moniker) to the forums. I note that the above is your first post and you are correct, your username and forum password are your purchase serial key.
One thing that you should always take note of when responding to a post is to look at the date of the previous post. In this case it was made about 18 months ago (February 2020) - I have been caught out by this myself. Unfortunately, posts that appear a long time (18 months or more) after the previous post are sometimes made by spammers as a "placeholder" for future spam so we are often suspicious of them.
Regards