Try x-safari-https:, as in:
x-safari-https://christiantietze.de/posts/2023/05/safari-for-mac-url-scheme
Which incidentally is a blog post about this :)
ctietze
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We have no Headers built phase and I did the opposite of @ivayale's solution and disabled the new build system integration (which previously was enabled):
defaults write com.apple.dt.XCBuild EnableSwiftBuildSystemIntegration 0
As always in Xcode, try toggling all toggles etc.; there seems to be neither sense nor reason behind the actual value. What matter is you change it :)
Same problem since I switched to an M1 Mac.