How to configure the values as path(URI) variable in target endpoint(upstream url) without using pluggins in kong gateway

Hello Everyone… I have a requirement like need to pass the URI path parameter values as headers via postman and its dynamically pic the values at runtime for upstream url.
Example:

—httbin.org/customers/{appname}/web/v1

as per the above example need to pass the appname value as string(.i.e. customer,employee).
Headers: appname=customer

And also, How we can set request.headers variable in service endpoint.