See here for Chapter & Ad Break Requirements.
While editing (or entering) your title:
Click the dropdown arrow next to the video file you want to add breaks to.
From here you should be able to add time codes for ad breaks under “Chapters & Ad Breaks”.
Chapter 1 always starts at timecode 00:00:00:00.
Then enter the hours, minutes, seconds, and frames of the timestamp. Ex: HR:MN:SC:FF
Label the breaks for easy understanding of where they happen in your film.
It is best to place an ad break after a scene's end or before a major lighting change.
Try to avoid placing ad breaks in the middle of a monologue, dialogue, or action sequence.
Make sure your frames in the time codes are < your video's frame rate or you will not be able to save.
Each Chapter has three fields:
Name - name of the chapter.
Time - timecode of the start point of the chapter. It's also the time location where the advertisement break is inserted.
Thumbnail Time: timecode of the thumbnail. The frame at this exact time location is used as the thumbnail image for this chapter on some streaming services.
Example
