Representing a jwt token as a formData parameter

Hey! 

I'm writing a 2.0 swagger file. It has a POST path, which consumes x-www-form-urlencoded and includes a JWT token as a parameter.

This token, before it's signed, should contain a JSON object. 

The problem is I can't find a good way of describing this in the swagger file. Essentially this token is a string, and as such should be represented as - in:formData, from what I understand, but in this case there's no way to use a $ref to describe all the object's properties before it was signed. 

I can leave the schema definition of the JSON object and just refer to it in the token parameter description, but I was wondering if there's a better way? 

Thanks in advance!