I assume that:
a) You already have a My Disney Experience account
b) You ticket information is already in your
MDE account
c) I haven't done a park reservation yet so I don't know about that part
d) This is a Magic Band which has never been assigned to anyone else.
You have to enter your MagicBand information into your My Disney Experience account, and assign it to yourself (as opposed to anyone else you have in your MDE account).
On a computer:
Go to DisneyWorld.com
Scroll past the COVID warnings and click on MY DISNEY EXPERIENCE
Sign in with your Disney user name and password
Hover your mouse over MY DISNEY EXPERIENCE
Click on MY MAGICBANDS AND CARDS
Scroll down past your name and the names of everybody else on your account
Then it will say: Link a MagicBand. Don't See a Purchased MagicBand? If you purchase a MagicBand and don't do in-store set-up, you'll need to link the MagicBand ID Number to your account.
Click on LINK
Under MagicBand, click on SELECT
Type in the MagicBand's ID number
(I can't take you to the steps past that because I don't have one to link)