woocommerce-subscriptions-custom-field.php

<?php

/**
 * Add a custom text field to a variable subscription product
 * @since 1.0.0
 */
function prefix_display_fields( $loop, $variation_data, $variation ) {
	$custom_field_value = get_post_meta( $variation->ID, 'my_custom_field', true );
	?>
	<div class="variable_subscription_pricing show_if_variable-subscription">
		<p class="form-row form-row-first form-field show_if_variable-subscription _subscription_my_custom_field">
			<label for="variable_subscription_my_custom_field[<?php echo esc_attr( $loop ); ?>]">
				<?php
				// translators: placeholder is a currency symbol / code
				echo esc_html__( 'My Custom Field', 'text-domain' );
				?>
			</label>
			<input type="text" class="wc_input_my_custom_field" name="variable_my_custom_field[<?php echo esc_attr( $loop ); ?>]" value="<?php echo esc_attr( $custom_value ); ?>" placeholder="<?php echo esc_attr_x( 'e.g. Hello', 'example text', 'text-domain' ); ?>">
		</p>
	</div>
	<?php
}
add_action( 'woocommerce_variable_subscription_pricing', 'prefix_display_fields', 10, 3 );

/**
 * Save the custom field
 * @since 1.0.0
 */
function prefix_save_product_variation( $variation_id, $index ) {
	if ( isset( $_POST['variable_my_custom_field'][ $index ] ) ) {
		$variable_my_custom_field = sanitize_text_field( $_POST['variable_my_custom_field'][ $index ] );
		update_post_meta( $variation_id, 'variable_my_custom_field', $variable_my_custom_field );
	}
}
add_action( 'woocommerce_save_product_variation',  'prefix_save_product_variation', 20, 2 );