Input route parameters in WebSocket API with VPC Link integration

I have a WebSocket API with a VPC link integration. When supplying the EndPoint URL I would like to input a route parameter (e.g. /users/{userId}/info). However I don't seem to be able to substitute this for an actual value. I have tried to use the available variables such as $request.body.userId and $input.body.userId. I have also tried to supply the variable by mapping it using request templates, but nothing seems to work. Does anybody know if it at all possible and if so, how?