listsubscribe API Issue

listsubscribe API Issue

I'm having trouble successfully POSTing to the listsubscribe endpoint.

The error I get back is:
  1. {"code":"2001","message":"Please enter correct XML(or)JSON format","version":"1","uri":"/api/xml/listsubscribe","status":"error"}

Here are some of the different string I have tried posting for the contactinfo field:
  1. {"firstname":"William","lastname":"Last","contact_email":"myemail@gmail.com"}
  2. {"First Name":"William","Last Name":"Last","Contact Email":"myemail@gmail.com"}
The main potential issues I see are:
  1. The API doesn't see to explain the contactinfo field format anywhere. What fields are available? Do I use the Field name? Display Name? Field ID? Something else?
  2. The sample provided in the API docs uses the Field Name but no quotes around the fields. Are quotes not allowed in the JSON? I would be very surprised because that's not valid JSON and my JSON library won't even let me do that.
  3. I'm screwing something else up and not noticing it.
Any input would be appreciated.