Something to understand about Lightroom... is that the originals are never changed. So you don't actually apply a watermark to the image "in" Lightroom. When you define a watermark, you can optionally have that watermark applied when you "export" the image.
Open Lightroom
Pick any image.
Right-click on the image to open the pop-up menu.
Mouse down to "Export ->" and when the fly-out menu pops out pick "Export..." (there's "export with previous" which is just a short cut that says "export this image using the exact same settings I used when I exported my last image" (in other words you don't get to change anything).
In the export panel you'll see lots of options... for example if it's a RAW image you probably want to convert to JPEG. Maybe you want to re-size it to a new resolution (I do if I'm sharing it online so that the image doesn't exceed 1000 pixels along it's "longest edge").
But keep scrolling and you'll eventually come to "Watermark" and you'll see a selection menu. Lightroom comes with a "sample" watermark, but you can (and probably will) build your own.
Pick the selection menu and you'll notice one of the choices is "Edit watermarks..."
This opens a new window. It has your image and you're in the watermark editor. It shows the placement of your watermark.
You can import an image (PNG or JPG ... note that PNG's can have transparent backgrounds, JPEGs cannot. If you want an "image" as a watermark then maybe you build it as a PNG with a transparent background.)
You can also build a watermark based on just text .. pick the found, style, color, and loads of other options.
I tend to reduce the opacity of my watermarks so they don't try to steal the attention away from my image. E.g. you could decrease the opacity to maybe 20-30% so that it's a mostly transparent watermark and shows up as a ghost in the corner of the image.
You can set your "anchor" position (e.g. if you want it in the lower right corner).
When you're happy ... DON'T click "Save" ... instead look at the upper left corner (above your photo) for the pull-down menu. Pick "Save current settings as new preset..." and give it a name.
I have a watermark for each year ... so I have my 2019 copyright watermark, but I also have my 2018, 2017, etc.. If you had just clicked "save" then it would overwrite the sample watermark and I prefer to preserve my old watermarks and create new ones.
The watermark only shows up in your EXPORTED images ... you wont see it on any image when you view it INSIDE Lightroom.
Having done all of this ... select all the images you want to share, click "export" and just make sure you tick-on the watermark setting of your choice (your newly named watermark will now be a choice on the selection menu in the watermark category.)