Can Safari search websites using aliases?

Safari does not have a keyword mechanism integrated with Google search.

Google and DuckDuckGo search both support the site: keyword, and DDG supports ! bang syntax for adding specific sites into the search.

You can add a text replacement for some short string (@github, for instance) and that’ll substitute in apps everywhere, and that keyword can be configured to insert (for instance) the site:github.com text for use in Safari and either Google or DDG.

Here is the macOS text substitution mechanism:

Replace text and punctuation in documents on Mac - Apple Support

Here is a Github user search showing DDG bang syntax (which could then be added as a text replacement), and here assuming your forum nick matches the nick of a Github user:

A text replacement for just the URL itself and not involving a search or a !bang would be more direct, obviously.

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