https://www.ableton.com/de/help/article/vst-plugin-midi-output/
Tutorial
Live tracks containing VST instrument plug-ins provide a MIDI input but an audio output. Therefore it’s not possible to record the MIDI output of a plug-in in this track directly. This is possible via track routing.
Load a VST plug-in into a MIDI track.
Create another MIDI track.
In the Input Type chooser (“MIDI From”) of the created track, select the track containing the VST plug-in.
In the (lower) Input Channel chooser, select the VST plug-in.
In case you want to route the MIDI signal back into the plug-in, do the following (this can be used for example to record the MIDI ouput of Native Instruments Maschine):
In the Output Type chooser (“MIDI To”), again select the track containing the VST plug-in.
In the (lower) Output Channel chooser, select the appropriate plug-in MIDI channel.
Use this MIDI track to record the MIDI output of the plug-in.
Create more MIDI tracks to send MIDI data to different MIDI channels