Infusionsoft custom field example. Event Espresso 4.

ee_infusionsoft_custom_field_example.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function ee_infusionsoft_save_my_custom_questions( $is_contact_data, $ee_attendee ) {
    if( $ee_attendee instanceof EE_Attendee ) {

        //get the last answer this attendee provided to the question with admin label 'custom_question'
        $custom_question_answer = EEM_Answer::instance()->get_one( 
          array( 
            array( 
              'Registration.ATT_ID' => $ee_attendee->ID(),
              // change custom_question on the next line to match the admin label in EE > Registration Forms > Questions
              'Question.QST_admin_label' => 'custom_question' 
            ), 
            'order' => 'DESC' 
          ) 
        );
        if( $custom_question_answer ){
          // change _CustomQuestion on the next line to match the Infusionsoft custom field
            $is_contact_data[ '_CustomQuestion' ] = $custom_question_answer->pretty_value();
        }
    }else{
        EE_Error::add_error(
          sprintf( 
            __( 
              'ee_infusionsoft_save_my_custom_questions was not called with an EE_Attendee but a %s', 'event_espresso' 
            ), gettype( $ee_attendee )
          ), 
          __FILE__, __FUNCTION__, __LINE__ 
        );
    }
    return $is_contact_data;
}
add_filter( 
  'FHEE__EED_Infusionsoft__save_infusionsoft_attendee__extra_attendee_data', 
  'ee_infusionsoft_save_my_custom_questions', 
  10, 
  2 
);