dynamic-default-value-checkbox-full.php

<?php

/**
 * Wrap hooks for a specific form.
 *
 * @param int $form_id
 */
add_action( 'nf_get_form_id', function( $form_id ){

  // Check for a specific Form ID.
  if( 21 !== $form_id ) return;

  /**
   * Change a field's settings when localized to the page.
   *   ninja_forms_localize_field_{$field_type}
   *
   * @param array $field [ id, settings => [ type, key, label, etc. ] ]
   * @return array $field
   */
  add_filter( 'ninja_forms_localize_field_checkbox', function( $field ){

    if( 'checkbox_1527003278102' == $field[ 'settings' ][ 'key' ] ){
      // Change the `default_value` setting of the checkbox field.
      $field[ 'settings' ][ 'default_value' ] = 'checked';
    }
    return $field;
  });
} );

dynamic-default-value-checkbox.php

<?php

/**
 * Change a field's settings when localized to the page.
 *   ninja_forms_localize_field_{$field_type}
 *
 * @param array $field [ id, settings => [ type, key, label, etc. ] ]
 * @return array $field
 */
add_filter( 'ninja_forms_localize_field_checkbox', function( $field ){

  // Change the `default_value` setting of the checkbox field.
  $field[ 'settings' ][ 'default_value' ] = 'checked';
  return $field;
});