Was it clear ? I dont know the right way to apply a fillet on the (red circle) area. If someone could help me I would be happy.
No, it was not all that clear. Are you wanting to apply the fillet to only the area circled in red? and leave the rest of the tangent edge un-filleted?
You can do something like this -
- but there is not a smooth transition. That was done by cutting where I wanted the fillet to stop, then making simple copies of the part and the cube used to cut and making a common, then after the fillet, fusing the Fillet part and the common part back together. When you make a Fillet on an edge, it will follow the edge through it's tangents to the end.
It would also help if you would post the file with the part in question. There may be other ways to accomplish what your wanting to do.