How to Receive Request Body of webhook using .NET webservice


I have a webhook set up that calls a webservice I have setup to recieve the json string. I’ve tested the webservice manually and it works.

However when I do a “Test” I get a message in my Workflow History:
Webhook failed: Server error
Server responded with error code 500

This is my WebMethod:
Public Function ConvertMarketingQualifiedToLead(json As String) As Integer
Dim postedData As String = "ConvertMarketingQualifiedToLead " & json
Return 200
End Function

Is the interface wrong? What parameters should I be using?
Thank you for your help!