Making a Button the Default Button in Cocoa

A default button pulses, and its action message is invoked when the user presses Return.

To mark a button as the default, set its key equivalent to Return with `setKeyEquivalent:`
```objective-c
[myButton setKeyEquivalent:@"\r"];
```

ref: https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Button/Articles/MakingaButtontheDefaultButton.html