I’ve just signed up with this ‘test@test.com’ and ‘test’ on my example website and added the first name of ‘test’.
So just try logging in with the same credentials and it should work fine for you.
Or you can make up your own if you want and try it with other names.
You will also need to make sure that any user has an actual entry in the first name field of their My Account page as shown here.
If you have just used the default Wix signup window then only the email address and password will be collected, so this field in the Wix Members app collection will be blank and return no value.
Also with above, if the new site member does not add anything to their My Account page as shown above, then this field in the Wix Members app collection will stay blank until they add an input themselves.
If you use the Wix custom window or your own Corvid signup lightbox, then you can add other user inputs for capturing first and last name etc and then these fields will be filled when the user signs up.