Some add-ons are not compatible to CRM custom fields, so to use information from CRM custom fields we've added a 'Custom fields links' menu in CRM administrator menu:
First create Select (Single choice) custom field with the name you want and add it to needed screens. Then go to CRM administrator menu / Custom fields links and link records from Companies or Contacts directories with previously created Select field and click on 'Add' button.
Click on 'Import records from CRM directory' and wait till the process is finished. Import is required only once and after that records will be added automatically if new companies or contacts are created in directories.
Now the company or contacts names can be selected from the dropdown list of Select (Single choice) field.
In the "Contacts" link line you can find "Properties" button that opens the dialog to link contacts to companies select field.
Choose companies select custom field to be able to select contacts that are listed in the company that was selected in the issue in companies select field. Don't forget to make records import after that.