Comment on page
When you use YayMail Pro with Checkout fields plugin (Flexible Checkout Fields by WPDesk or Checkout Field Editor by ThemeHigh), you can add a custom field into your Checkout page.
In Checkout page, it looks like the image below.
Then please make a real order with that field, and you can have its data in YayMail.
After seeing the shortcode generated, you can copy it and put it anywhere in an email.