@Steffen_Abel I just checked the pricing page of Freshdesk and the Custom Apps feature is available only from the “Pro” plan of Freshdesk.
If you are using Freshdesk as a product for your support helpdesk, you would have to upgrade your account to Pro to use Custom Apps in your account. You can contact your Technical Account Manager or anyone who has reached out to your Freshdesk to get to know more about it.
If you are building apps for other Freshdesk customers, or integrating your product to publish on our Marketplace for mutual customers to benefit, or build and publish Marketplace apps to Freshdesk Marketplace acquire users, please get your account trial on the Pro plan to use your account to build, test, and publish Custom apps and Marketplace apps. Refer to this article for the steps.