You’ll need to add a section on the track and set its range/duration. The section points to the animation asset.
Something like:
animSection = animTrack.add_section()
animSection.set_range(make_range_seconds(0, length))
You’ll need to set the animation asset on the section as well. I can’t remember off the top of my head, but it’s probably something like:
animSection.set_editor_property(“Animation”, yourAnimimationAsset)
Take a look in here for examples: Engine\Plugins\MovieScene\SequencerScripting\Content\Python