It'll look something like this. I'm assuming that you're not using an email template. Go into source mode on the editor to add this. You'll need to get the record type of your custom record from the url (rectype=186) in my case. custrecord_co_internal_id is the field name on my record that references the custom record. Adding the .id to the end of the field name is what gets you the internal id of that record.