Remove (or customize) 'Search' from help menu

I have found the way to remove the search bar (but not to customize it).

Just assign a menu that is not used to the help menu:

NSMenu *unusedMenu;
unusedMenu = [[NSMenu alloc] initWithTitle:@"Unused"];

NSApplication *theApp;
theApp = [NSApplication sharedApplication];
theApp.helpMenu = unusedMenu;

The documentation mentions this in the helpMenu property of the NSApplication class.

You probably don't want to get rid of that search bar, since you can still use it to search for menu items!

As I'm sure you know, this search box will only show Help Topics if your app comes with an Apple Help Book, which can be made by following Apple's documentation.

I'm afraid I don't know of a way to override the search bar's behaviour, but if you don't want to write documentation for your app, I think it would be better to keep the search bar, even if you can't search your forum for help.

You're looking for NSUserInterfaceItemSearching protocol. Return a single search result item and use it to open your custom URL.

- (void)searchForItemsWithSearchString:(NSString *)searchString resultLimit:(NSInteger)resultLimit matchedItemHandler:(void (^)(NSArray *items))handleMatchedItems
{
    handleMatchedItems(@[searchString]);
}

- (NSArray *)localizedTitlesForItem:(id)item
{
    return @[[NSString stringWithFormat:@"Search for '%@' on my website", [item description]]];
}

- (void)performActionForItem:(id)item
{
    // Open your custom url assuming item is actually searchString
}