I think this is only possible from the iOS / Android SDK. I did a lot of digging, including through their iOS SDK, to see if I could grab the URL. Here's what this link looks like when sending from iOS to the messenger.
This link will actually work to open your Messenger (but the image will be blank if you have nothing in the Poster folder
fb-messenger-platform-20150714://broadcast?pasteboard_type=com.messenger.image&app_id=XXXAPPIDXXX&version=20150714
Please note that the iOS SDK has saved the image com.messenger.image. They mainly store the image in a cardboard box. You can read about it here https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIPasteboard_Class/ )
If anyone has any insightful or hidden API, please let me know. Trying to post videos and animated gifs and nothing worked yet.
This is the only way to send the iOS SDK, I'm still digging all the code comments to find out if I can find any idea of ββother parameters that may or may not be available.
Sorry for the more than bad news, if you are not building an iOS APP then this is easy.
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