How can I have a single choice populate a Long Text Entry when checked

Hello,

In order to achieve the functionality you are looking for, you first have to define the calculation values in your single choice element like this:

Step two is setting up a condition that will grab the single choice fields value and populate the long text entry element with it, here's a screenshot of my test condition:

Here's the result:

If you require any more help with this, or any other matter, please don't hesitate to contact us! 

Thanks!