Default text for custom message in draft order invoice notification

Hi,

In the custom message for a draft order invoice notification, I’d like a default custom message already populated to avoid having to type one each time. What code can I use in the template for this to happen? I can’t seem to figure it out and any help would be appreciated.

I’m also interested in learning how this can be accomplished. I’ve looked at several of the custom invoice apps, but none of them include the checkout link contained in the default “send invoice” from a draft order.