Garmin has an API for 3rd party developers, so if you want to connect any 3rd party service, you need to ask the owners to develop and offer the API interface. It means this forum is not the right place to ask. You should ask at Lifesum.co, it is in their own interest to support available hardware platforms. Garmin already has what's needed.
They do not need to develop any API (Application programming interface), they just need to plug into the API that Garmin already provides, and exchange data with Garmin Connect.
However, they will have to pay license fees, and that may be the real reason why they did not do it yet. MFP certainly pays Garmin rather important fees for the use of the API, but it still seems to be profitable for them. Hard to tell whether Lifesum cares about such advantage.
So one app points to the other since years. Is there any benefit for the user of rejecting responsibility? Both Garmin and LifeSum are super tools. They would be perfectly complementary. But why do I have to note down the calories that I burned and the calories that I eat outside the two tools and calculate the difference manually? And that in the year 2022? So isn't this just a really bad feature prioritization of the product owners on both sides as it is obvious that this feature is necessary?
Thanks e7andy! I am perhaps a bit old school but I had the impression that myFitnessPal puts my data on servers outside Europe and somehow I don't have a good feeling for my data privacy. I know that this is usual but I would feel more comfortable in a world where I as a user decide what happens with my data rather then having to accept the polcies of the providers. Especially when it comes to my most important data such as health data. But perhaps you are right and I have to accept or leave it. Thanks again.