There are no options for an Administrator to set.
When at the top level of the forum if you click in the search box and enter your search term it will search throughout the forum
If you are in a sub-forum then as you enter your search term you also get the choice to search only that sub-forum (and all sub-forums below it) or searching "Everywhere" (as if you are at the top level of the forum(.
At all levels in the forum there is also a link to "Advanced search". That screen can you you work out the syntax to use for special searches.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.