How do I make a box transparent in Markup? I take a screenshot using shift+Command+4 and right click on it and choose the option "Markup" and then draw a rectangle box but how do I make it transparent so that content underneath is visible?

Here is the screenshot showing what I am taking about:



I tried several options but couldn't find a way to make these boxes transparent.Can someone help me on this?
Choose the fill colour button on the toolbar (red filled square in your screenshot), then open the colour palette with the button at the bottom of the popover, and set the opacity on the colour using the slider or entering a percentage (100% is fully opaque, 0% is fully transparent).