I) Once in the Add page, click on the Marketplace Button for TikTok in the Red Box.

II) You will be redirected to this page, where you should select one of the 3 countries that are included. 

III) After selecting the region, you can sign up for a seller account in the red-highlighted box.

IV) If you already have an account, you can sign in using your phone number and password.

V) After that, follow the site instructions to authorize the permissions needed to integrate.