How to add a "Custom clip" dynamically from script?

In Timeline, when we want to add a clip dynamically we can use something like:

var track = timeline.GetOutputTrack(trackIndex);

var clip = track.CreateDefaultClip();

AnimationPlayableAsset animationPlayableAsset = clip.asset as AnimationPlayableAsset;

animationPlayableAsset.clip = animationClip;

But, that's for the pre-defined tracks from Unity Timeline Engine. If I created a Custom Playable using the Default Playables Wizard, how are we supposed to add a clip on that custom track?

For example, we can create a custom playable called "Dialogue" using Default Playables Wizard. And it will generate classes like "DialogueClip", "DialogueTrack" automatically. Well, adding a clip could be easily right-click on the track and "add Dialogue Clip", but how could we supposed to do this from script?