ok, here come the solution (thanks to Aliaksey)...
If you want to connect TheThingsNetwork to Flespi, you need to use the MQTT-Client Functionality of flespi.
TTN is a mqtt broker, so you need to configure in flespi a "channel" from type MQTT.
the USERNAME in flespi for the mqtt client (channel) is the APPLICATION ID in TTN (here "hilltronic".)
The PASSWORD for the connection is the "default access key". (you can create an additional key for security reasons...) it starts with "ttn-account-v2xxxxxxxxxxx".
URL for flespi = eu.thethings.network:1883 (if you use EU gateways).
Than come the next; you need to subscibe! to a mqtt topic.
This you can do by adding a new topic in flespi.
If you want to subscribe to ALL objects/devices in one "application-id" you just do it like this:
hilltronic/devices/+/up (where hilltronic=the application id and the "+" sign asks for all devices and the "up" the upstream.
(for downlink i will write another text after i understood this :-)
The last thing is to use the parameter conversion (see the ttn3.png),
If i am deeper into this, i will also update it,.
And you should create a device in flespi to see the results you are receiving...,
After you have the date in Flespi, voila you can forward it to wherever you want.