ActionTag - prefill a custom field input

preFillCustomField.js

// see https://developers.everyaction.com/action-tag#callbacks-alterfill
// assumes your Custom Field ID is 1234
function populateCustomField(args) {
  if (args.fill_source !== 'FastAction') {
    return args;
  }
	
  if (!args.fill_dict['CustomField_1234']) {
    args.fill_dict['CustomField_1234'] = preFilledCustomFieldValue();
  }

  return args;
}

function preFilledCustomFieldValue() {
  // determine the appropriate value for this person...
  return 'foo';
}

var nvtag_callbacks = nvtag_callbacks || {};
nvtag_callbacks.alterFill = nvtag_callbacks.alterFill || [];
nvtag_callbacks.alterFill.push(af);