I've made custom emails for customers' quotations and invoices, with a link to sign up in our portal. I can't get {object.signup_url} to insert the signup link. {object.signup_url} just inserts a blank. Is there another variable I should be using? 

I recall the original quote/invoice emails having a link that'd go to the signup page, and automatically insert the customer's email and name (eliminating the chance of them putting in an email that doesn't match up). Right now, I have to keep merging accounts in order for customers to view their documents.

An example of the code I'm using:

<a href="${object.signup_url}">Test link</a>

Thanks for any help.