How do I advise the airline who my customs clearing agent is?

When your goods arrive at destination, the airlines handling agent should contact you to advise when your goods are waiting to be customs cleared. In the event that you aren't contacted by the handling agent, you can contact the airline's office and ask who the handling agent is and their telephone number.

Advise the airline that any documents need to be passed to your preferred clearing agent, and that the goods should be released to their 3-letter badge code.  Your agent should give you this.

You may have to send this in writing to them, either via fax or email.