From Xojo Documentation

Property (As Boolean )
aPushButton.Default = newBooleanValue
BooleanValue = aPushButton.Default

Supported for all project types and targets.

If True, the default button indicator is added to the button and the Return and Enter keys are mapped to the button. The default is False.

Sample Code

This code sets the PushButton as the default button. It is in the Open event of the control.

Me.Default = True