You should use a UIFocusGuide that encompasses the UIButton you want to focus. Make the UIFocusGuide as wide as the collectionView and tall enough to cover the button. Make the preferredFocusView the button.
UIFocusGuide *topButtonFocusGuide = [[UIFocusGuide alloc] init];
topButtonFocusGuide.preferredFocusedView = myButton;
[self.view addLayoutGuide:topButtonFocusGuide];
[self.view addConstraints:@[
[topButtonFocusGuide.topAnchor constraintEqualToAnchor:myButton.topAnchor],
[topButtonFocusGuide.bottomAnchor constraintEqualToAnchor:myCollectionView.topAnchor],
[topButtonFocusGuide.leadingAnchor constraintEqualToAnchor:myCollectionView.leadingAnchor],
[topButtonFocusGuide.widthAnchor constraintEqualToAnchor:myCollectionView.widthAnchor],
]];
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