How to send SFMC SMS via external API?

Question

I'm trying to trigger SFMC REST API to send SMS from Postman. Below is the steps:

1. Created a Outbound SMS content in Mobile Connect (with the following properties)

   • Template type: Outbound - API Triggered
   • Next Keyword: OPTIN
   • Message: Hello world message
   • Message Status: Active/Scheduled
   • API Key: xxxx

2. Trigger SFMC REST API using Postman client

   • Call https://auth.exacttargetapis.com/v1/requestToken (with client ID and Secret) to obtain the access token
   • Call https://subdomain.rest.marketingcloudapis.com/sms/v1/messageContact/xxxx/send with the following json

   { "mobileNumbers": [ "85296xx57xx" //My HK number ], "SendTime": "2018-12-11 04:08" }

   • It responses a token ID: yyyyy
   • Call https://subdomain.rest.marketingcloudapis.com/sms/v1/messageContact/xxxx/deliveries/yyyyy to check the status delivery. It returns unknown token message

After performing the above steps, I still not able to receive the SMS. Is there a way to troubleshoot this?