Reference an UTXO of a transaction that has not been yet mined
The UTXO is referenced as an outpoint(TXID, Index) in the input of your time-locked spending transaction.

So, when creating your time-locked transaction, you need to know the TXID of the (unconfirmed) transaction you are spending. 

Simply double+sha256 and swap endianness of the over-the-wire transaction serialisation to get the TXID you wish to reference. 

This TXID is a unique identifier (assuming no TX malleability). The outpoint you are referencing in your spending transaction is not affected by the confirmation of said UTXO.