You want to add a draft watermark to a dashboard as shown in the following...

[References: https://help.tableau.com/current/pro/desktop/en-us/dashboards.htm https://help.tableau.com/current/pro/desktop/en-us/dashboards_create_layouts.htm https://help.tableau.com/current/pro/desktop/en-us/dashboards_create_watermarks.htm, To add a draft watermark to a Tableau dashboard, you would use an image object. You can overlay this image across your dashboard, possibly with some transparency, to achieve the watermark effect without interfering with the readability of the dashboard content., ]