ASM/WAF policy - Parameter value type was determined to be "XML value" but really it is "HTML"

You could define /aa/bb on the allow URLs and add to it a Header Based Content Profile:

Request Header name: Content-Type

Request Header Value:* application/x-www-form-urlencoded * 

Request Body Handling: Form Data

Profile Name: N/A

Then you can create a parameter text and select the parameter value type to XML value. Then use the XML default content profile