chocco Hello! You'll first need to ensure your video is in either OGV or WEBM format. Then to import the video into your project, you can either drag the file onto the RPG in a Box application window, or click the "Import" button on the top toolbar and browse for the folder where it's located.
This documentation has a bit more info about this process, as well as a list of supported file types:
https://rpginabox.com/docs/doku.php?id=import_resources
Once the video is imported into your project, you'll use the "Play Video" node in your script. For example, if you'd like the video to play when the player interacts with an object, you'd include it in the script assigned to the "Character Interacts" trigger event. Hopefully that helps!