For this, you will have to use Extension:FlaggedRevs. For custom namespaces you will have to further set the setting: $wgFlaggedRevsNamespaces
.
For example, in your LocalSettings.php:
// Enable the extension
require_once("$IP/extensions/FlaggedRevs/FlaggedRevs.php");
// Set FlaggedRevs for Project namespace
$wgFlaggedRevsNamespaces = array ( 4 );
Use Manual:Namespace#Built-in namespaces for namespace numbers.
Although in T30662 and T970 (Miraheze) for $wgFlaggedRevsNamespaces
namespace names are being used instead of their numbers, but the documentation only mentions namespace numbers. It probably should work using either method.