Adds an event handler that runs when the element receives focus.
An element receives focus through user actions, such as clicking and tabbing, or programmatically, using the focus( ) function.
Note: onFocus() has no effect when applied to RadioButtonGroup elements.
handler(event: Event): void The name of the function or
the function expression to run when the element receives focus.
This method doesn’t return any custom errors, but may return standard errors. Learn more about standard Wix errors.