The fixture list will by default not show a 'buy ticket' button. You will need to go into the fixture to enter the URL.

Go to articles, put the opposition in the filter at the top and find the fixture then click on it. 

Click on the fixture details tab, then scroll down until you see the ticket link box, then enter URL and save. This will add a 'buy ticket' button to the website. To take it off deleted the link and click save.

Ticket link