Consider that the "main menu" is at top right corner of the stackedit.io page.
In the first step, log in with a Google account to synchronize all data.
Add via this link.
Redirect URI type https://stackedit.io/oauth2/callback;Confidencial;api option;Click on main menu (StackEdit icon) -> Accounts -> Add a GitLab account.
GitLab URL type https://gitlab.com/oauth/applications/<application-id-in-the-url>;Application ID paste the Application ID copied from https://gitlab.com/oauth/applications/<application-id-in-the-url>;Ok.main menu-> Synchronize -> Save on GitLab;Ok.