There are several ways to link a company card, in all cases the custom field of the CRM Company type should present on the issue screens.
Selecting a Company
There are two ways to select a company.
Select a company in the issue field:
Create an issue directly from the company card (in case you have access to the companies in the CRM) – the company will be selected during the issue creation:
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: