I just make my own and turn it into a Photoshop brush.
When you say "desktop lightroom", are you using Lightroom Classic? If so, I'd recommend posting your question on the following forum...
This topic has been moved to the Lightroom Classic forum.
Also look at the related topics to the right. Your question may have been answered.
But the original question is not a Lightroom CLASSIC question, it is a Lightroom question. This is not the correct forum either.
A watermark can only be added to an image at time of Export.
So in LrC you first have to select the images you want to add a watermark to, Click the Export button and in the Export dialog window near the bottom you will see section that is for adding a watermark to the new file that is created at the time of exporting.