How do I open an URL with a Zendesk custom field content as parameter

To open an URL from your zendesk ticket sidebar you can configure the Deeplinks App URL like this:

• let's assume you have a JIRA-ticket ID stored in a custom field with the ID 3600000999
• deeplink configuration
  • https://<yourjirainstance>.atlassian.net/browse/{{ticket.customfield_3600000999}} 

Please find a list of Placeholders with examples here: 

Deeplinks App - Example placeholder list