The CRM app provides a bunch of custom fields to connect the CRM entries to Jira issues. You should add those you'd like to use, configure them and add to the relevant screens (read: Adding a custom field (Atlassian documentation).
Read the particular custom field documentation to know about field configuration.
The following fields use options and work similarly to the Select List type fields: CRM Select custom field, CRM Companies select field, CRM Company, CRM Contact, CRM Custom Directory, CRM Multiple Custom Directory, CRM Named Select Custom directory, CRM Products Select, CRM Single Product Select.
But unlike the Select List, you cannot enable/disable options within the list.