add-variation-field.php

function variation_settings_fields_rohartikel( $loop, $variation_data, $variation ) {
    // Text Field
    woocommerce_wp_text_input(
        array(
            'id'          => '_rohartikel[' . $variation->ID . ']',
            'label'       => __( 'Rohartikelnummer Halle', 'woocommerce' ),
            'placeholder' => '002015971001',
            'desc_tip'    => 'true',
			'readonly' 	  => true,
            'description' => __( 'Enter the custom value here.', 'woocommerce' ),
            'value'       => get_post_meta( $variation->ID, '_rohartikel', true )
        )
    );

}

function save_variation_settings_fields_rohartikel( $post_id ) {
	// Text Field
	$text_field = $_POST['_rohartikel'][ $post_id ];
	if( ! empty( $text_field ) ) {
		update_post_meta( $post_id, '_rohartikel', esc_attr( $text_field ) );
	}
}
// Add Variation Settings
add_action( 'woocommerce_product_after_variable_attributes', 'variation_settings_fields_rohartikel', 10, 3 );
// Save Variation Settings
add_action( 'woocommerce_save_product_variation', 'save_variation_settings_fields_rohartikel', 10, 2 );