You can place the watermark on the images at the time when you export the images from the Lightroom.
Check the help document below for the details.
I don't want to export , i want watermark the photos in the collections synchronized in the cloud and share them.
we wont be able to do that.
Sadly, the only way you can do this is by exporting watermarked copies of your files, bringing those into your catalogue, and sharing them.
Yes, it is ridiculous.