You should have UOAssist running at this point. If you don't, start it up. Now log all the way into the game with any of your characters on any of the official OSI servers. If you aren't in a safe place you can log right back out as UOAssist will save the seed it made during the logon procedure. Press the '?' button in the bottom right corner of UOAssist (circled in red below). If you are running UO in full screen mode you'll need to either switch to windowed mode (alt-enter) or hit alt-tab to see the UOAssist screen.
After press the '?' button you should see the About screen. Press the button labeled "Registration ..." on the About screen. This should bring up the following screen:
The seed is found in the box right near the top. In the screen shot above the box is blank and has a gray background. On your screen you should see something like ^7ac~,tUaq,Yu or something funny looking like that. If the seed box says '<You must login to Ultima to obtain a seed>' then log out of the game and then back in again. Seeds are very 'sensitive' so it is important that you use copy and paste to move the seed from UOAssist's registration dialog to the spot on the trial registration form in your web browser. Again it is VERY important to use the exact seed, using the copy (select the seed then press ctrl-c) and paste (ctrl-v) method ensures accuracy (copy is ctrl-c and paste is ctrl-v in most Windows applications, if ctrl-v doesn't work in your browser see if you can find Paste on the Edit menu and use that).
You might note the blue box near the bottom of the above screen shot. That is where you'll need to put the key once you receive it. Don't forget:)
A few extra notes on what the seed is or isn't. The seed does not contain your account name and/or account password. It contains an identifier received from the servers so I can make unique seed/key pairs. The information in the seed does not allow access to your account or access to your private information from OSI's registration site. You can change your account name or password at any time and it won't affect your seed or key. Also if you have multiple machines you will notice that seeds for the same game account are different. This won't affect your key at all and is normal. If you have multiple game accounts please read this .