Did you try adding HTTP headers ?
The username and password supplied for HTTP Basic Authentication is ultimately an HTTP header field.
As such, each SOAP test request in soapUI can be configured with a HTTP Basic Authentication username and password.
When the test request is run an “Authorization” header is added to the HTTP envelope that contains the SOAP request.
You can do this through the Interface Viewer. To open the Interface Viewer, right-click on a WSDL operation in the project explorer tree on the left-hand side of the soapUI interface and choose “Show Interface Viewer”.
This will open a screen with four tabs: Overview, Service Endpoints, WSDL Content, and WS-I Compliance. The HTTP Basic Authentication credentials are set through the “Service Endpoints” tab. For each endpoint in the list you can specify what username and password to use. Once specified, all of the test requests on the WSDL operation (the one you selected to open the Interface Viewer) using the specific endpoint will automatically use the username and password you entered.