While you cannot use an "Annotation", you could customize the label for the reference line. See below example.
On Right click reference line, choose Edit. In the Edit window choose "Custom" for label:
You can then combine text, as well as values in the field.
However, do note that by default the labels will be at the bottom of the viz. Since you have a viz that one needs to scroll, it may be better to format the Label to be on the top. This can be done by choosing the "format" option on right click of the reference line, and setting the desired option for "Alignment".
Happy to help!