Locktime is an optional part of bitcoin transactions. It sets the earliest time a transaction can be mined into a block. It is possible to use locktime with Trezor to make sure that a transaction is locked until a specific block height or a point in time.
How to use locktime with your Trezor
1. Calculate the block you would like to add your transaction into as: current block height + number of blocks to wait for adding transaction.
You can find the current block height on this blockchain explorer for bitcoin and this blockchain explorer for litecoin.
2. Click on Add locktime in the right bottom corner of the transaction and add the desired block into the locktime box.
3. If you add a future block as a locktime in your transaction, an error will appear on your screen. No need to worry, Trezor Wallet just can't broadcast the transaction because of the locktime. You can now copy the raw transaction in hex format and save it. It is necessary to broadcast the transaction after the given block is reached, e.g. with external broadcast transaction tool.