SPFX adding event listener to field

I have a form customizer project that uses typescript and react.

I have a choice field which renders as a dropdown and one of the choices is to leave a comment.

If the user selects the 'leave a comment' choice I wan to show a multiline text field beneath the dropdown that allows the user to enter their comment.

This would be a very simple task in straight javascript. Just changing the visibility style of the textbox within the onchange event of the dropdown, but I cannot figure out how to accomplish this task in the form customizer project.

Does anyone have an example of how to do this or any other information that would be helpful?

Thanks!