Heartbeat 7.9.0 Http Send Request Headers not working

Trying to send the user information which is required as part of the Header

Http Monitor

# Request settings:
  check.request:
    # Configure HTTP method to use. Only 'HEAD', 'GET' and 'POST' methods are allowed.
    method: "GET"

    # Dictionary of additional HTTP headers to send:
    headers:
      'User': 'Domain\username'

    # Optional request body content
    #body:

Can you try using double quotes?

Which part ? The Key or Value or Both ?

"User": "Domain\username" -> The Slash in the domain is not escaped and causes an issue

Neither work