If it's a message on OF and you have access to a computer it's really easy to download the source files without having to install any software or extensions or anything.
I've heard you can pull source files from mobile if you use Aloha browser, but I've never tried it and I don't know if it still works.
go on a computer, (assuming your browser is google chrome) right click anywhere on the page, click Inspect, click on the video, and there will be a downloadable link on the right, it'll look like "src=(link)," download that yourself and upload it to bunkr
I’ve used Aloha browser and it works well for this. It’s easy to directly download and copy it using it. Probably the best option to use with the least amount of difficulty.