Hi, if you are publishing a SOAP service from Mendix app, then it is very easy to enable authentication.
- Open your published web service
- In settings tab, you can find the authentication
- You must create a WebService User which is also explained in https://docs.mendix.com/refguide/published-web-services/#311-authentication
- With this setup, consumer of the service, can use the WSDL, User details and call the service succesfully.
Hope this helps