There are two ways a company card can be linked to an issue. In both cases, the custom field of the CRM Company type should present on the issue screens.
Selecting a Company
Automatically Defining a Company
If set up, the company will be auto-populated according to the Reporter's company and will be updated on the Reporter's change:
Managing a Company In an Issue
In both cases, if you have the relevant permissions, you can click on the company name to browse and manage the company information in the pop-up: