Hello! To "attach" a bone to a vertex, you need to first bind the mesh to the bones that need to influence it: Weights - Spine User Guide: Bind
then select the weights tool, choose maybe direct mode, select the 3 vertices, then raise the influence of the bone you want to 100%.
At this point that bone should be able to control it.
We cannot see your slack attachments because we'd need to be logged in to your workspace, so it would be more convenient to upload the gif to an external service like giphy.
What do you plan to do with that bone? did you want to move it up, scale it, or something different?
You can change the language of your interface to French, then press F1 to see useful popups that explain how Spine works, but we don't have docs in French, I'm sorry.