ninja_forms_pre_population_callback.php

add_filter('ninja_forms_render_options','ninja_forms_pre_population_callback', 10, 2);
function ninja_forms_pre_population_callback($options, $settings) {
  
  // target only the field with this key
  if( $settings['key'] == 'my_field_key' ) {

    // write the query to fetch the data
    $args = array(
      'post_type' => 'post',
      'post_status' => 'publish'
    );

    $posts = new WP_Query( $args );

    if ( $posts->have_posts() ) {

      $options = array();

      while ( $posts->have_posts() ) {

        $posts->the_post();
        
        // $options is the variable which contains tha values rendered
        // we will use the post title as lable and the ID as value
        $options[] = array(
          'label' => get_the_title(),
          'value' => get_the_ID()
        );
      } // endwhile
    } // endif

    wp_reset_postdata();
  }

  return $options;
}

So I have two select fields in my Ninja Form. Please note that my google sheet is a list of products and corresponding brands. First one displays a list of brands from the google sheet and the second one needs to display the products based on the brand selected. I have the Conditional Logic add-on installed but I am not sure how to get the selected brand in the functions.php file without the need to submit the form.