Where do we configure the path parameters in MF SV? How to capture path parameters dynamically?

If have an URL to test recurlysubscriptions/.../billingInfo. 

I want to know where do I setup the 'account_id' parameter in MF SV? Till recurlysubscriptions/accounts, I made it base path and, I have tried setting the /{account_id}/billingInfo in UriPath in request header, but it is too volatile. I want to restrict the account_id to 12 characters. 

When user pass the account_id in the get request, I want to capture it and copy it to the corresponding field in the response. Please let me know how to do it.