If you have the username and password you can access the web service in the browser. Then you could copy the wsdl into a file and use the file as the source for your web service.
You can take a look here for documentation on how to import webservices in Mendix. At the bottom of that page there is also information on how to deal with authentication.
You have use the 'call webservice' action in a microflow, then you can select your imported web service operation.
In the request header tab page (in the microflow action) You can choose to either use a domain to XML mapping (mapping), by filling in the values for each parameter (simple), a combination of domain to XML mapping and filling in the values for each parameter (advance), or you can create the request yourself with the relevant parameters (custom).