Subplots - Annotations for row=1, col=2

I have created a subplot as follows:

fig = make_subplots(rows=1, cols=2, column_widths=[0.85, 0.15])

To both i have added horizontal lines (control limits) and i would like to create annotations for them.
The annotation for the fist chart (row=1, col=1) work fine with the folliwng code:

fig.add_annotation(x=end_date, y=UCL,
text=“UCL”,
showarrow=False,
xanchor=“left”,
font=dict(
color="#EC6500"
),)

But i am not able to do the same (an annotation) for the sencond plot (row=1, col=2).
I have tried it withe following code, but without any sucess:

fig.add_annotation(x2=SpecHeight, y2=32,
text=“USL”,
showarrow=False,
xanchor=“left”,
font=dict(
color="#EC6500"
),)

Is there any way, how to define the x2 and the y2 axis?