Swagger: How to add default headers in every request?

For example if i have:

paths:
 \some:
   post:
     description: Sample request
     parameters:
      - in: header
        name: firstHeader
        schema:
          type: string

How can I add this header in every request, without repeating this code? I've tried to reference as object after parameters, but in that case it won't be part of the headers of the request.