How to Set Required Fields

// executionContext is passed as function parameter
var formContext = executionContext.getFormContext();

// Set field as required
formContext.getAttribute("fieldname").setRequiredLevel("required");

// Set field as recommended
formContext.getAttribute("fieldname").setRequiredLevel("recommended");

// Set field as optional
formContext.getAttribute("fieldname").setRequiredLevel("none");

Source: setRequiredLevel (Client API reference)