How do I add Custom Y-axis labels??

To apply label to existing line series select the series and then use the right arrow to scroll through the data points. When the last point is selected apply data labels.
I have done both Series Name and Y value labels.

Or you can add another series, I did one for "-3s" which has no values expect for last data point. You can then apply data labels to whole series but only last data point will be displayed.