When a future ticket is created, in most cases the original ticket will be closed and Zendesk will automatically create a 'clickable' link between the original ticket and the future ticket using the native follow-up functionality, however, this is not true if the original ticket is still open.
This article explains how to create a link to the original ticket within the ticket description or initial comment using a ticket template.
Create a ticket template with an HREF link to the original ticket
1. Open the Future Tickets app 
from the left colored pane in Zendesk.
2. Click ticket templates in the main left bar and select add ticket template at the top right. If you already have an existing ticket template, you can edit this instead.
3. Select the comment and insert a HREF to create a link to the original ticket. Below is an example of how this could look:
Original Ticket: <a href="{{ticket.link}}">#{{ticket.id}}</a>4. Fill in the rest of the ticket template and select create/update ticket template.
Now whenever a future ticket is created and this ticket template is used, a 'clickable' link will be on the ticket description or initial comment.
Note, that this is only a suggestion. You can adjust the wording according to your preferences. The only requirement is that you use a HREF and use the ticket.link placeholder between the quotations.