Check a question group box by default. For the Event Espresso 4 event editor. Primary Question group ID 3.

my_custom_checkboxes_for_primary_question_groups.php

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

add_action( 'AHEE_event_editor_questions_notice', 'my_custom_checkboxes_for_primary_question_groups' );
function my_custom_checkboxes_for_primary_question_groups() {
  echo '<script>jQuery( 
      "#espresso_events_Registration_Form_Hooks_Extend_primary_questions_metabox input[value=\'3\']"
    )
    .prop( "checked", true );</script>';
}

usage.md

Usage instructions

• Add the code to your custom functions plugin.
• Find the question group ID for the question group that you want to always have checked for the Primary Registrant
• Change the 3 in the above code to match your question group ID