What are the options for choosing ad campaign URLs?

[ParaDockz]

Good evening friends, as of now, I am trying to advertise. I have reached a certain stage. I do not really understand what I need to do in the section where the options for the advertisement URL are. I need your help to overcome this. I do not have my own website. Can I use my Facebook page? Additionally, I only want my ad to be seen on mobile devices. Do I need to fill in the boxes for PCs and tablets under the devices tab, or should they remain empty? Please help.