Product Enquiry form with Ninja Forms - https://www.damiencarbery.com/2018/05/product-enquiry-form-with-ninja-forms/

add-ninja-form-to-product-description.php

<?php
// Include a Ninja Forms form in a product page. Different forms for simple and variable products.
add_filter( 'the_content', 'dcwd_add_ninja_to_all_products' );
function dcwd_add_ninja_to_all_products( $content ) {
    if ( class_exists( 'woocommerce' ) && is_product() && is_main_query() ) {  // Check suggested by: https://pippinsplugins.com/playing-nice-with-the-content-filter/
	    global $product;
	    if ( 'simple' == $product->get_type() ) {
			ob_start();
			Ninja_Forms()->display( 1 );  // Equivalent to shortcode: [ninja_form id=1]
			return $content . ob_get_clean();
		}
		if ( 'variable' == $product->get_type() ) {
			ob_start();
			Ninja_Forms()->display( 2 );  // Equivalent to shortcode: [ninja_form id=2]
			return $content . ob_get_clean();
		}
    }
    
    return $content;
}

convert-variations-to-options.php

<?php
// Populate the specific list box with product variations.
add_filter( 'ninja_forms_render_options', 'dcwd_populate_ninja_forms_select_for_variations', 10, 2 );
function dcwd_populate_ninja_forms_select_for_variations( $options, $settings ) {
	if ( 'select_product_attributes' == $settings[ 'key' ] ) {		
		global $product;
		$options = array();

		// Examine the variations.
		foreach ( $product->get_available_variations() as $variation ) {
			$variations = array();
			// Record the attributes of each variation.
			foreach ( $variation[ 'attributes' ] as $attribute ) {
				$variations[] = $attribute;
			}

			// Then assemble this combination of attributes into a single variation option.
			$options[] = array(
				'label' => esc_html( implode( ' ', $variations ) ),  // Readable version e.g "S White".
				'value' => esc_attr( implode( '-', $variations ) ),  // HTML entity version e.g. "S-White".
				'calc' => 0,
				'selected' => false,
			);
			
			// If viewing a submission get the submitted value in case it is no longer an option in the form.
			if ( is_admin() && array_key_exists( 'post', $_GET ) ) {
				$post_id = absint( $_GET[ 'post' ] );
				$selected_value = get_post_meta( $post_id, '_field_' . $settings[ 'order' ], true );

				// Check whether the selected value is already in $options (either part of the form or added above).
				$key = array_search( $selected_value, array_column( $options, 'value' ) );
				// Only add the selected value if it's not present.
				if ( false === $key ) {
					$options[] = array(
						'label' => $selected_value,  // The original display label is not available.
						'value' => $selected_value,
						'calc' => 0,
						'selected' => true,
					);
				}
			}
		}
	}
	return $options;
}

product-enquiry-for-ninja-forms.php

<?php
/*
Plugin Name: Product Enquiry form with Ninja Forms
Plugin URI: https://www.damiencarbery.com/2018/05/product-enquiry-form-with-ninja-forms/
Description: Product enquiry form for simple and variable products.
Author: Damien Carbery
Version: 0.1
*/


// Include a Ninja Forms form in a product page. Different forms for simple and variable products.
add_filter( 'the_content', 'dcwd_add_ninja_to_all_products' );
function dcwd_add_ninja_to_all_products( $content ) {
    if ( class_exists( 'woocommerce' ) && is_product() && is_main_query() ) {  // Check suggested by: https://pippinsplugins.com/playing-nice-with-the-content-filter/
	    global $product;
	    if ( 'simple' == $product->get_type() ) {
			ob_start();
			Ninja_Forms()->display( 1 );  // Equivalent to shortcode: [ninja_form id=1]
			return $content . ob_get_clean();
		}
		if ( 'variable' == $product->get_type() ) {
			ob_start();
			Ninja_Forms()->display( 2 );  // Equivalent to shortcode: [ninja_form id=2]
			return $content . ob_get_clean();
		}
    }
    
    return $content;
}


// Populate the specific list box with product variations.
add_filter( 'ninja_forms_render_options', 'dcwd_populate_ninja_forms_select_for_variations', 10, 2 );
function dcwd_populate_ninja_forms_select_for_variations( $options, $settings ) {
	if ( 'select_product_attributes' == $settings[ 'key' ] ) {		
		global $product;
		$options = array();

		// Examine the variations.
		foreach ( $product->get_available_variations() as $variation ) {
			$variations = array();
			// Record the attributes of each variation.
			foreach ( $variation[ 'attributes' ] as $attribute ) {
				$variations[] = $attribute;
			}

			// Then assemble this combination of attributes into a single variation option.
			$options[] = array(
				'label' => esc_html( implode( ' ', $variations ) ),  // Readable version e.g "S White".
				'value' => esc_attr( implode( '-', $variations ) ),  // HTML entity version e.g. "S-White".
				'calc' => 0,
				'selected' => false,
			);

			// If viewing a submission get the submitted value in case it is no longer an option in the form.
			if ( is_admin() && array_key_exists( 'post', $_GET ) ) {
				$post_id = absint( $_GET[ 'post' ] );
				$selected_value = get_post_meta( $post_id, '_field_' . $settings[ 'order' ], true );

				// Check whether the selected value is already in $options (either part of the form or added above).
				$key = array_search( $selected_value, array_column( $options, 'value' ) );
				// Only add the selected value if it's not present.
				if ( false === $key ) {
					$options[] = array(
						'label' => $selected_value,  // The original display label is not available.
						'value' => $selected_value,
						'calc' => 0,
						'selected' => true,
					);
				}
			}
		}
	}
   return $options;
}