Its possible to use a macro for webchecks and provide a username and password but this is not working 100%:

If the macro parameters are empty, they are still being sent which results in a 401 statuscode response rather than 200.
It is not possible to use a macro for the "authentication" part, to switch between None/Basic/NTLM.

This can be fixed two ways:
1) Do not authenticate if both fields are empty (e.g. contain a macro that is not set on none of the levels)
2) Make it possible to use a macro to switch between the authentication type.