How to annotate faceted line graph

I have a px.line graph that is faceted on the row level and I would like to annotate each sub graph as such:
I have my x value anchored through a callback.

How can I update my x and y value here:

fig.add_annotation(x=?, y=?,
            text="Text annotation with arrow",
            showarrow=True,
            arrowhead=1)