Currently, it is not possible to add Linked Issues in the customer portal.
When adding the field in the request type field, the Show button will be greyed out, it says "This field type can't be shown in the customer portal".

As a Service Desk administrator, I would like to configure the visibility of linked issues to appear in the Customer Portal as it is helpful in some use cases.

Tentative Workaround

As a way to allow customers to visualize whenever a new issue link is created, the following automation rule can be used. This will result on new public comment being added on each of the linked issues, pointing to the other:

1. Set the Trigger

• Select the trigger Work Item linked.
• This trigger fires whenever an issue is linked to another issue.
• You can specify the link type (e.g., "blocks", "relates to") or leave it as "Any link type".

2. Add the Action: Comment on Work Item

• Click New action and select Comment on work item.
• Enter your comment. You can use smart values to reference issue fields, link type, or the key of the linked issue. For example:

This work item is linked to {{destinationIssue.key}}.

3. Add a Branch for Related Issues

• To comment on the linked (destination) issue as well:
• Add a Branch rule / related issues component.
• Select Destination issue.
• Inside the branch, add a Comment on work item action with your desired message, such as:

This work item is linked to {{triggerIssue.key}}.

4. Finalize

• Uncheck Prevent Duplicates on each of the comment actions, to allow for new links to be commented if more are added later on
• Ensure that the Rule Actor may comment on the issues on each project (Also take into account Issue Security)