How to enable Submit button only if user has provided input in Sugarcube 2

Hi,
I am using Sugarcube 2.0.

I want user to:
1. Select an option from multiple radiobuttons (without automatically checking any radiobutton by default)
2. Enter text in a textarea (this can be optional or mandatory based on game setting)

I would like to enable the SUBMIT button only if they have selected one radiobutton, and entered text in textarea (if is mandatory as per setting).

How do I do this?

Thanks in advance