If this is happening for your vanity URL, then you have to set up an SSL certificate for your custom domain. 

SSL options for custom domains are available from the Blossom Plan. Please write to us at  support@freshdesk.com,  and our support team will help you in obtaining the SSL certificate.. 

Once you receive the email, please follow the instructions to confirm your request for an SSL certificate. 

Click here to read more on Configuring a custom SSL certificate for your support portal.