In this case, creating the fillet with the API is relatively easy, but the tricky part is identifying the edge you want to fillet. Interactively, as a user familiar with the geometry, it's very easy to click on the desired edge. But how would you describe to someone else that doesn't know anything about the model which edge should be filleted? That's essentially what you need to do in your program; figure out some rules that result in finding the desired edge and then write the code using those rules.
Maybe something like finding a cylinder or a certain diameter that is the furthest of any others in the negative X direction, and then finding the edge of the cylinder that is not a circle. That logic is based on the topology or the ability to get the edges of a face, and the faces that an edge connects. It also uses geometry to find cylinders of a certain radius and their position in space.
Another approach is to find the edge that is at a specific X,Y,Z location in space. Likely, you don't have this information, but in cases where you do, that can be an efficient way to find geometry.
Here's a link to a class I presented several years ago that goes into more detail about this.
https://www.autodesk.com/autodesk-university/class/Understanding-Geometry-and-B-Rep-Inventor-and-Fus...
Once you've found the edge you should be able to use code like that shown in this sample to create the fillet.
https://help.autodesk.com/view/fusion360/ENU/?guid=GUID-7105bf44-52db-4067-8d7a-afd73c24e954
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Brian EkinsInventor and Fusion 360 API Expert
Website/Blog:
https://EkinsSolutions.com