Write a letter to USCIS stating that your asylum case is pending. But understand that your asylum case must be pending when the initial registration period (First TPS registration) started for TPS. State when you filed asylum and provide the receipt number of asylum and a photocopy of it. That is all USCIS is asking for. Send it to them and you will be all good.
Address of USCIS
Dear USCIS Officer,
I filed for asylum on xxx-xxx-xxx which is still pending. My receipt number for pending asylum is xxx-xxx-xxx. I have attached the copy of the receipt, please have a look.
Let me know if you need any more information.