If you want to send a video reminder in the app, just follow these steps.
Open the document that you want to send a reminder from
Click on the three dots next to the document name
Choose "Send video reminder"
Write a message that you want to send with the video
Press "Record video" and start recording
Stop the recording when done and choose "use video" in the bottom right corner. The video should then upload
Press "Send" and it is done!