How do I send a video from my iPad to Dropbox - when not a camera upload?

Hi, I can see how to transfer my camera uploads from ipad to Dropbox.

However - I've a video that's not in camera uploads... I watch it through the "videos" icon at base of my ipad screen, ie through the iTunes Store app. It was purchased from a company and sent to me that way. How can I move it to Dropbox please? I can't see any way to move it into camera uploads - which seems to be the only folder that Dropbox gives me an option of uploading from.

Thanks!

Chris