Why am I getting Status "404 Not Found in Postman”

 I am trying to trigger a Trigger Send with a simple payload using Postman, however the TS is not being sent. Note: No response body is generated, the only thing that it shows is "404 Not Found"
POST: {{rest_url}}messaging/v1/messageDefinitionSends/key:TS_ExternalKey/send { "To": { "Address": "EmailAddress", "SubscriberKey": "SF_Contact_Key", "ContactAttributes": { "SubscriberAttributes": { "ResetURL" : "https://www.google.com/" } } }, "Options": { "RequestType": "SYNC" } }
In the Email, I am using the following AMPScript:
set @resetURL = ResetURL Link

What could be the possible reason for this behavior?