I got a Fritz!DECT 400 working! What I did was use the Fritz Smart Home interface (not the HA Fritz integration) to create a webhook. The webhook can then be used in HA for triggering automations/scripts, etc.
In the Fritz! Smart home settings for the Fritz400, create a template. You have the option to enter a URL. The format you want is: http://192.xxx.xxx.xx:8123/api/webhook/webhook_id
Obviously, enter your HA IP address and change “webhook_id” to whatever you want to call it. Next is important, click the drop-down for “Additional Settings” and change the method to “POST”. Once your template is done, you need to create an automation next. This basically links the button press of the switch to the template you just created. The 400 switch let’s you have two automations, short press, and long press. I used both so I have two templates created because I need two webhooks so HA knows the difference.
You can now use a webhook as a trigger. You only need to enter the webhook id, not the full URL inside HA.
Tony