Lookups can be added into hotspot links in rich text signatures.
Example 1: a hotspot was created over the First & Last Name, and the mailto link contains a lookup to the person's email address.
Example 2: a hotspot to add social media icons
Select the image, and create the hotspot over the image. Then in the value field, enter the URL which can contain a lookup (if all or part of it is to be calculated), as shown below.