I've set up MiniBB working properly. But on "search" page, I want to set one forum as the default one to search as "where" item.
I've tried to use Javascript "selectedIndex" but it does not work. Is there any "option" value to achieve this?

Basically you need just to specify forum ID in the link which points to the Search form. Like

index.php?action=search&forum=11

for example. It would be possible to create a non-core-destructive solution for this... or you may modify load_header() function of bb_functions.php and modify this directly, but changing the core file.

Thanks.
By the way, I'd rather not "destruct" the core with copyright.

Then you will just replace standart {$l_menu[1]} search menu button in templates/main_header.html to let's say {$searchMenu}

And then paste in bb_plugins.php the following code:

/* Custom search menu */

if($action!='search') $searchMenu="<a href=\"https://www.minibb.com/forums/{$indexphp}action=search&amp;forum=FORUM_ID\">{$l_menu[1]}</a> {$l_sepr} "; else $searchMenu='';

/* --Custom search menu */