Okay, now the issue is "400: Bad Request" that means the user authentication is successful. Bad request means that the URL you are passing is incorrect. Please check it once.
For testing, you can save the URL being passed in a string attribute and check for typo errors(if any).
The error is 401 unauthorized, It is an authentication issue.
"Login FAILED: unknown user 'Admin'."
You could try logging in with MxAdmin credentials and check the result.