I understand you want to register your Fire TV Stick using an activation code through Amazon MyTV.
After you start the setup on your Fire TV Stick, you’ll be guided either to sign in directly on the device or to complete registration using an activation code shown on the TV screen.
With standard registration, Fire TV Stick asks you to sign in using an existing Amazon account or create a new one. When prompted, you can enter your Amazon account email address and password directly on the TV screen to complete registration.
In some cases, Fire TV Stick requires an additional verification step.
An activation code appears on your TV screen along with instructions to complete registration on the Amazon website:
Write down the activation code displayed on your Fire TV Stick.
Using a phone or computer browser, go to the website shown on your TV screen (such as Amazon MyTV or the code page).
Sign in to your Amazon account if asked.
Enter the activation code in the provided field and select Continue.
Note:
The Amazon account you sign in with on the website will be the account used to register and link your Fire TV Stick.