REST APIs: custom HTTP headers vs URL parameters
Solution 1:
The URL indicates the resource itself. A "client" is a resource that can be acted upon, so should be part of the base url: /orders/view/client/23.
Parameters are just that, to parameterize access to the resource. This especially comes into play with posts and searches: /orders/find?q=blahblah&sort=foo. There's a fine line between parameters and sub-resources: /orders/view/client/23/active versus /orders/view/client/23?show=active. I recommend the sub-resource style and reserve parameters for searches.
Since each endpoint REpresents a State Transfer (to mangle the mnemonic), custom headers should only be used for things that don't involve the name of the resource (the url), the state of the resource (the body), or parameters directly affecting the resource (parameters). That leaves true metadata about the request for custom headers.
HTTP has a very wide selection of headers that cover most everything you'll need. Where I've seen custom headers come up is in a system to system request operating on behalf of a user. The proxy system will validate the user and add "X-User: userid" to the headers and use the system credentials to hit the endpoint. The receiving system validates that the system credentials are authorized to act on behalf of the user, then validate that the user is authorized to perform the action.
Solution 2:
I would only use a custom header when there is no other way to pass information by standard or convention. Darren102 is explaining the typical way to pass that value. Your Api will be much more friendly by using typical patterns verse using custom headers.That's not to say you won't have a case to use them, just that they should be the last resort and something not already handled by the HTTP spec.
Solution 3:
When do you use...HTTP headers in the request part of a REST API?
Authentication: GUIDs, basic authentication, custom tokens, etc. e.g., Basic Authentication with a Guid token for REST api instead of username/password
If you get involved in passing tokens or other authentication-like information between domains covered by PCI-DSS or other security rules you may also have to bury parameters because some regulations explicitly require authentication elements to stay out of URLs that could be trivially replayed (from browser histories, proxy logs, etc.).