User manual:Using locktime
1. Calculate the block you would like to add your transaction into: current block height + number of blocks to wait for adding transaction.
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 cannot 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 an external broadcast transaction tool.