To enable vertical scrolling for a radio button group form item, you must set the 'ControlGroupVertical' property to 'true'. A sample code snippet is provided for your reference:
this.fm.registerHandler("cbxpreinitialize", function(fm, event, fieldname, value) { return { 'TRANSFER_FROM_RADIOBTNGRP': { ControlGroupVertical : true; } }; }); //Here, TRANSFER_FROM_RADIOBTNGRP is the Item ID of the radio button group form item.