acording with this code the name of the field is campaign not accountType, so in the URL you should have ?campaign=ProspectFreeClass1,
what my script will do is look at all the parameters passed in the URL (eg: campaign). then check if there is a mautic form field with that name (or name=“mauticform[campaign]”) and set the value of the field in this example ProspectFreeClass, the user is free to change the form field (unless you define the field as “readonly” or “hidden” , and the value will be send to mautic server once the user submits the form