@ewok21
This is an excellent beginner case. To begin you could configure a principle flow like:
message from HomeKit -----> Change node where you configure the message properties -----> To Somfy node
In the message from Homekit you get the data you need, position 50, but the message object is not structured so you can use it directly to send it to Somfy
Using the Change node you can create the necessary message properties that is required by the Somfy node
Then share your flow (use export function) here and describe what nodes you are using to get more and better help