Also, it would be helpful to allow the content of the textarea to be preloaded, as we can do e.g. on
https://en.wikipedia.org/wiki/Special:ExpandTemplates?wpInput=EXAMPLE
In this case, "wpInput" is the value of the "id" and the "name" attributes of the field, so for consistency, on
https://test.wikipedia.org/wiki/Special:AbuseFilter/test
and
https://test.wikipedia.org/wiki/Special:AbuseFilter/examine
the parameter should be "wpTestFilter" and on
https://test.wikipedia.org/wiki/Special:AbuseFilter/tools
it should be "wpTestExpr".

Not sure if this is "easy" enough for a new contributor who needs to find the right place in the code. Also, comment 1 might be a separate request.

Well, I added the other parameter here as well because by looking into the related ExpandTemplates code
https://github.com/wikimedia/mediawiki-extensions-ExpandTemplates/blob/f277894539831417039d97ba4f38a44519fe17a2/ExpandTemplates_body.php#L20-L52
I think it is just a few extra lines in a single part of the code, probably this:
https://github.com/wikimedia/mediawiki-extensions-AbuseFilter/blob/f7ed4cffeec645124f6cc3111f35d00d9029f839/special/SpecialAbuseFilter.php#L21-L37

Change 408076 had a related patch set uploaded (by Matěj Suchánek; owner: Matěj Suchánek):
[mediawiki/extensions/AbuseFilter@master] Allow preloading text areas using URL parameter

https://gerrit.wikimedia.org/r/408076

Change 408076 merged by jenkins-bot:
[mediawiki/extensions/AbuseFilter@master] Allow preloading text areas using URL parameter

https://gerrit.wikimedia.org/r/408076