Just put your var in the window context
window.myVar = prompt("Enter a Variable", "");
To reuse it later just get an named element by ID and replace innerHTML
(Naming is done under ID_Information-> Unique Element ID)
var elm = hypeDocument.getElementById( 'your_id_name_goes_here' );
elm.innerHTML = window.myVar;
Change VAR- and ID-names to suit your needs and naming preferences.
There are no security mechanisms in place but for simple purposes without storing this for other users you should be fine. But to be save I recommend reading up on: