Hey @srk123
Welcome to Meta 
From what you describe, I would assume that the assign plugin is enabled.
You can Reverse engineer the Discourse API to find out how a request to assign a topic looks like.
As a customer, you can also send an email to the support inbox team@discourse.org from the email connected to the account on the forum you moderate (or you can message the @team group here at Meta. If you signed up here using the same email address, they can also identify that you are a customer).