You can choose a video cover when creating a post:
1. after you've attached a video, click / tap Select Video Covers;
2. choose the source (from the attached video, upload media);
3. move the green box to the frame you like and click / tap Save;
4. give it some time to process the result, you should see the confirmation that the cover is selected;
5. if you're done, click / tap Create Post;
💡 Covers help you choose the best image for your thumbnail, and a good thumbnail can attract more fans.