How to add searchable field for existing ticket in Service Manager Form and/or Request Type?

[Nirav Patel]

I am trying to create a new request type and a form in Jira Service Manager for user requests. One of the field I need to provide is Existing Jira Issue where a user submitting a ticket can look up existing ticket from the field and provide link.

for example: user should be able to look up existing issues and select in this field

I have tried using "Linked Issues" field under the "Request Type" but it appears to come up as Hidden by default and I cannot seem to unmark it (it's greyed out)