Realistic HTX-100 Owner's Manual page 22

CONNECTING A CODE KEY
You connect a code key if you plan to transmit CW with your
transceiver.
1
2
22
Attach a shielded cable with a
1 / 8
inch (mini) plug (not
provided) to the code key.
Insert the code key's plug into the KEY jack on the
back of your transceiver.