All supported parameters will be shown in the popup automatically. Or there are no parameters required for that operation. Or the parameters from the webservice aren't supported.
Did you get any info or error messages when you were trying to import the wsdl??
Maybe you can post some the part of the wsdl that defines the operation.
Here is some additional information about which type of parameters the modeler supports:
Any complex (multi-level) xml structure. BUT this structure can't be mapped with the popup you show in the question. In that case you should press the button convert to custom body and you will see the xml structure which you will have to fill manually in your microflow.
In the 2.5 release you will be able to create a mapping so you won't have to create the complete request.
All elements such as binary, the any or anytype element are not supported. Binary will be supported in the 2.5 release. But the any or anytype can't be supported since this element can contain any value. Just a simple value or a huge complex xml structure. That way the modeler and XAS won't know what to expect, which can cause a lot of problems therefore it is prohibited to use those elements.