I have been trying to get an access token so I can then make a callout via the API. Here is what I have done in Postman:

Endpoint: https://platform.ringcentral.com/restapi/oauth/token

I have entered my Username and Password both with these being base encoded 64 and not encoded. This time they are not encoded. I have entered them under the Authorization tab

I have added in Headers Content-Type : application/x-www-form-urlencoded
 

In Body tab selected the x-www-form-urlencoded selected:
    - Entered grant_type: urn:ietf:params:oauth:grant-type:jwt-bearer
    - Entered assertion : eyJo…. lots of middle stuff]...NDz that is encoded base 64

Method is POST
 

Result is

{
  "error" : "invalid_request",
  "errors" :  {
    "errorCode" : "OAU-345",
    "message" : "Invalid assertion signature"
  } ],
  "error_description" : "Invalid assertion signature"
}

​​​​​​​What am I doing wrong???