If you have setup a custom login URL for the live site WP admin, then you will need to add custom login URL for staiging site also, kindly follow the steps below
Step 1: Go to WP Plugin Settings -> Staging Tab and enter the custom login link, as shown in the image below
Step 2: Click Save Changes.
Step 3: After the staging process visit the staging site staging site custom login, eg: https://mysite/staging/custom-login, to login to the staging site admin.
