Hi there!
Thanks for your query.
Actually there is no default way to do that with SP Page Builder addons.
However, you can always use custom css to do that with before/after element.
Here is some sample code:
#Your_Addon_ID:before {
height: 50px;
width: 10px;
background-color: maroon;
position:absolute;
}
Note: Path towards custom CSS:
Extensions>Templates>Styles>
Click on your current default template name then choose Template Options.
Then find custom CSS within the custom code.
https://prnt.sc/pu705g
https://prnt.sc/pu70zn
Hope this helps anyways!
Best Regards