To enable MTC transmission, first of all you need to toggle on the SYNC button around the bottom-right corner of the Composition view. A Composition cannot be synced to any source while MTC transmission is enabled.
While MTC transmission is enabled, scrubbing the playhead or jumping on the timeline results in an MTC Full Frame message to be sent with the current time. If you have any custom start time set in the MTC output settings (see below), the start time and the current time will be summed.
You can set up the MIDI destination, as well as the SMPTE time format and the Start Time.