It depends on what version you are using. In Lightroom Classic you can add all kinds of signatures from text based to graphical. This is done upon export. In Lightroom (cloud based), you do it also in export and tick the "show watermark" checkbox. If you are on a mobile OS (i.e. iPhone, iPad, or Android) , you should hit the little symbol to the right of the "export to camera roll", or Export to files and select "include watermark" where you can also costumize the watermark. The same setting is available in "export as" if you select tiff or jpeg. You obviously cannot include a watermark when you export to original or to dng.