Please visit this page for details about signing up for the Free Trial, and then follow the on-screen instructions to complete the process.

There is no registration code for this campaign.
Instead, permission to play the Free Trial will be allocated to the Square Enix account you enter on the launcher screen after completing the steps required to start playing.

However, please be aware you cannot play the Free Trial using a Square Enix account that already has a licence for either the retail version or FINAL FANTASY XIV beta test.

Yes, the Worlds you can choose to play on are the same in both versions. 
New character creation may be restricted on certain Worlds depending on the current population.

The client is the same for both versions so it is not necessary to uninstall it.
Please see below for details and the required steps for transferring your character from the Free Trial to the full version.

Yes, the character you create and level up in the Free Trial can be transferred to the full version of the game.
(Please note that you cannot transfer characters from the Free Trial to a service account that already has a Free Trial associated with it.)

Upgrading to the PlayStation®4/PlayStation®5 version
Be sure to enter the Square Enix account you used to play the Free Trial on the licence registration screen.
If you played the Free Trial on the PlayStation®4/PlayStation®5 version, you need to acquire the game on the same PSN account used to play the Free Trial.

Upgrading to the Xbox Series X|S version
Be sure to enter the Square Enix account you used to play the Free Trial on the licence registration screen.
If you played the Free Trial on the Xbox Series X|S version, you need to acquire the game on the same Xbox account used to play the Free Trial.

Upgrading to the Windows version
Players who wish to do so need to purchase a copy of the game, log into the Mog Station using the Square Enix account used to play the Free Trial and add the registration code to the service account with the Free Trial.

Please note that you can only transfer to the Steam full version if you signed up for the Steam Free Trial.

Once your service account is upgraded to a full service account on any platform, you will no longer be able to revert back to a Free Trial account.