This standard element function is not supported for RichTextBox.
Adds an event handler that runs when the input element receives input.
A text input receives an input event when a user changes its input value.
You can retrieve the input element's new value by getting the value from the
target property of the Event passed to the handler you set
using this function.
eventHandler(event: Event): void The name of the function or
the function expression to run when the input element receives
input.