In this example, there is a Radio button on the form (input 6). There are 2 choices (yes=0, and no=1). When these are chosen, the hidden location field will be updated with the email address defined in the dropdown location field (Here are the details for creating the location field - 5. Create a Form) This is what the example of the location field:
On the form, add HTML field and insert the code below. Change field 6 to your radio button field ID, field 8 to your hidden location dropdown field ID and 139 to your form ID. You will also need to change the email1 and email2 to the labels that you have chosen in the location field.
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<script>jQuery(document).on('change', 'input[name=input_6]', function() {
console.log('is working?');
if(jQuery('#choice_139_6_0').prop('checked') === true) {
jQuery("#field_139_8 option:contains('email1')").prop('selected','true');
} else if(jQuery('#choice_139_6_1').prop('checked') === true) {
jQuery("#field_139_8 option:contains('email2')").prop('selected','true');
}
});</script>