adding days to dates

I need to add 365 days to a certain date field.

In an other topic somebody wrote:
If you have a Date field in your Form for Start Date, add another date field(Expiry Date) in your Form and write a code in the On success of the form as follows....
input.Expiry_Date== input.Start_Date.addYear(2);

I am trying this, but I think I need a more detailed explanation. Where do I add this expression? And how?

Thanks for you help!