As the link provided by Dave doesn’t work anymore as of OS X Yosemite, I investigated a bit and found this updated version of his link:
macappstore://userpub.itunes.apple.com/WebObjects/MZUserPublishing.woa/wa/addUserReview?id=YOUR_APP_ID&displayable-kind=30
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As Dave already mentioned, this links to the App’s page and opens the "Write a Review" section. But the App Store doesn’t scroll down to the section, so on small screens the user thinks he’s only been taken to the normal App’s page. So I dug a bit deep and also found those links (same base URL), that don’t return a fully functional (App Store) page, but instead return content that is normally called via AJAX requests on an App’s page inside the app store:
/writeUserReview?id=YOUR_APP_ID&displayable-kind=30
– links directly to the "Write a Review" section content
/userRateContent?id=YOUR_APP_ID&displayable-kind=30
– links directly to the "Rate this App" 5-stars rating call
/saveUserReview?displayable-kind=30
- links to the page, that saves user-ratings
Note: On the last two links I removed some parameters, because otherwise they could be used to rate and review any app (the user has bought) without user interaction!
I don’t know if this is really helpful to somebody, but I wanted to write it down here, as somebody else might be interested in this.