How to use/call amazon connect API StartChatContact & StartContactChat to initiate the chat channel?

Question

I am trying to follow StartContactChat & CreateParticipantConnection to initiate the chat from third party applications e.g. Postman. i want to route the chat to agent on talk to agent message from API, the chat should be routed to the agent in amazon connect. StartChatContact's Request Syntax is given like this:

PUT /contact/chat HTTP/1.1
Content-type: application/json

{
   "Attributes": { 
      "string" : "string" 
   },
   "ChatDurationInMinutes": number,
   "ClientToken": "string",
   "ContactFlowId": "string",
   "InitialMessage": { 
      "Content": "string",
      "ContentType": "string"
   },
   "InstanceId": "string",
   "ParticipantDetails": { 
      "DisplayName": "string"
   }
}

I have done with it using URL: PUT https://connect.us-east-1.amazonaws.com/contact/chat and got the ParticipantToken and now trying to create the participant connect using CreateParticipantConnection - https://connect.us-east-1.amazonaws.com/participant/connection but keep facing the error:

{
    "message": "Unable to determine service/operation name to be authorized"
}

i have added the participant token generated by StartContactChat in Authorization --> AWS Signature --> Session Token as well as in Header but still the still getting the AccessDeniedExcetion.