Unfortunately, it's not technically possible to post a review directly on a user's behalf using Google/Facebook APIs. 

The customer will be shown a link to post the same review on that platform and their comments automatically copied to the clipboard. They can add the review in a couple of clicks, so it's the next best thing.
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Take a look at our demo EndoForm to try out that functionality (rate 3 or more stars to view links to other platforms): https://forms.endorsal.io/form/5dcb0a1537bb9809dbfeb5fd/
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