If you use a Wix-managed login for members, Wix automatically takes care of the login process. However, if you've created a custom login page, you need to indicate where Wix should send members to login. You do so by setting a login URL.
To set a login URL:
In your project dashboard go to Settings > Headless Settings.
From Headless clients, click the three dots to the right of the OAuth app you want to edit. Choose Settings to open the app's settings page:
Scroll down to the URLs section and go to the Login URL section.
Enter the URL to your custom login interface. When Wix needs to log in a member, the member will be redirected to the URL you entered.
Click Save to save your changes and return to the main Headless Settings page.