Can I add a link to the production description to show the signup form?

Yes, you can add a link to the product description of a specific product. To open the signup form when clicked your link needs to include the id 'BIS_trigger'

Note: This article is for adding the link to a single product at a time. If you would like to add a link to all sold out products by editing product template please see Add a custom button to the product page

To add your link:

1. From the Shopify admin find the product and click to edit.
2. In the description create a link. Enter the text for the link and highlight it. Click the Insert Link button.
3. In the dialog enter '#' as the Link to field and hit Insert Link to confirm.
4. Now switch to HTML view by clicking Show HTML
5. Find your link and add id="BIS_trigger" (just before the href attribute is a good place.)
6. You can now hit Save and view the link on your product page.