In ON1 Photo 10, ON1 Photo RAW 2017 and Photo Raw 2018, you will be able to add a watermark when you are using the export feature. Note: you will need to have a file of your watermark or logo already made in order to use this feature.
with the export feature, choose the "+" at the top right of ON1 Photo 10 and ON1 Photo 2017/2018 shown below:
From here, you will be given multiple options to choose from, choose "watermark".
Once you choose this, you should see the screen below in the right hand pane of program. You can change the location of your watermark and edit some of the settings right from here.