I want to add a custom field to a member's account so that I can add the name of another connected member.
But I would like the name of this second linked member to display as clickable to open the second linked member's Membercard.
To generate a hyperlink you need the user_id which is a second value in addition to the username. A custom field will only allow for one value. You could enter both the username and user_id as they appear in the URL:
That will allow you to build a link, but then you don't have the username as a separate value for the visible text. You could create a listener for a template hook to parse the value and separate the name and user_id, but that requires programming.