How to keep signup lightbox open after form submit?

I’m using a “Member Signup Form” to get user information. This form is a lightbox with a web form.
On the form’s submit button click event, I’m using a backend function to send and http request.
When the backend function returns, I want to display the response data to the signup form.
However, the lightbox closes immediately after the click event handler returns.
How can I keep the lightbox open until the user closes it with the ‘x’ button?
Thanks,
Tim