Incapable of Creating a Manual Line (Threshold) on a Box and Whisker Chart

May consider this as workaround:

1. Add a Threshold Series
You can simulate a threshold line by adding a new data series:
•    Create a new row or column with your threshold value repeated for each category.
•    Add this series to your chart as a line chart.
•    Format it to appear as a horizontal line across the box plots.
•    Right-click the chart → “Select Data” → Add your threshold series.
•    Then format the new series to use a line style and add it to the legend.
This method works well and keeps the line part of the chart, not just floating on top.
2. Use Error Bars
If your threshold is tied to a specific metric, you can use custom error bars to simulate a line:
•    Add a dummy series with values at the threshold.
•    Apply error bars that span across the chart.
•    Format the bars to look like a line.

3. Overlay with Combo Chart

•    Create a combo chart: box and whisker + line chart.
•    Use the line chart to plot your threshold.
•    Make sure both series share the same axis.
•    Format the line to match your desired threshold style.
This method allows legend integration and dynamic updates.