How do I include merge fields in a tracked link for a document or mailing?

Environment:

Mailing, Tracked link, Merge fields, Custom fields

Resolution:

Merge fields, including custom fields, can be used in tracked links that are included in documents and/or mailings.

The format should look like this:

Incident custom field using "custom_field" as an example:
http://www.mydomain.com/=$incidents.c$custom_field

Incident merge field using Reference Number as an example:
http://www.mydomain.com/=$incidents.ref_no

You can also pass multiple variables in the same URL by placing an ampersand (&) between them:
Example uses "custom_field" as a custom field from the incidents table and "first_name" as a standard merge field from the contacts table

http://www.mydomain.com/=$incidents.c$custom_field&$contacts.first_name

NOTE: If the contact associated to the record is changed after the tracked link is sent, merged data may not be available for the contact as expected.  For example, once the contact is changed from a to b, the link sent to contact a will not get the value from the merge field.

For more information on Tracked Links, see Answer ID 1848: Creating and using tracked links with mailings.