Help regarding uploading file from a external application onto a website built in Bubble

I am currently building an application in Unity Game Engine in which I need to upload a file(may be images or files of other format) from the application into a website I have built in Bubble. I couldn’t understand the way the documentation explained at “Introduction - Bubble Docs”. I am also attaching a screenshot of the specific section I am interested in. It would be very helpful If you could please suggest me a good tutorial on this subject.

Thank you,
Arin Kumar Poray