You can type in the search much like you would use Google or any other search field.
+CSPWeather- custom races with CSP/Sol weather integration enabled,
+LiveWeather- custom races with live weather enabled,
RaceConfig.MaxClients:<32>will list all custom races with less than 32 max clients configured.
Of course, you can combine all of the above things into one query! That might look like the following:
+practice +RaceConfig.Track:"Spa" RaceConfig.MaxClients:<32
Server Manager uses Bleve to provide searching. Bleve is incredibly powerful and offers a great amount of control over search queries. We've covered some examples of how to make the most out of searching here, but please refer to Bleve's Documentation for more detailed examples and syntax.