Problem sending POST request

I'm using a desktop REST client to send a request but I keep getting:

{
    "code": "INVALID_TOKEN",
    "details": {},
    "message": "invalid oauth token",
    "status": "error"
}

I'm sending an Authorization header as follows:

Zoho-oauthtoken 1000.54c1a8cbb00d2a0ed98350eed6.06b7f99a96ee9079291a2b5bea

The documentation was not clear on this but I'm sending the Client ID and Secret separated by a ".".

Any ideas?