Once that is saved, you can reference the Data Element in custom script (you were entering it in the right place). For example:
s.purchaseID = _satellite.getVar('name of your data element');
Right after that custom code, you may also want to use _satellite.notify('custom script fired', 1); or something like that for debugging purposes. _satellite.notify() is a protected version of console.log(). You can look in the console and make sure that script fired as expected.
Hope that helps. Let us know if that doesn't work for you.