How do you create a NSTouchBarItem that uses the remaining width in the Touch Bar?

I'm trying to create a NSButton that takes up the remaining space on the Touch Bar. My goal is to have a button similar to the "Search or enter website name" button in Safari.

I've looked at setting constraints with widthAnchor, but I'm completely lost at how to set it properly.

NSButton has an intrinsicContentSize that fits its content, and a default horizontal contentHuggingPriority of 750 (NSLayoutPriorityDefaultHigh), meaning it will strongly try to match that content width.

You can decrease that hugging priority (e.g. [button setContentHuggingPriority:1.0 forOrientation:NSLayoutConstraintOrientationHorizontal]) in order to make it more able to stretch away from that content, and in this case fill up the extra space in the touch bar.