A tracking link is a short URL on your own domain that records the click and then redirects to wherever you were sending people anyway. It is the earliest point at which anything can be recorded, and it works where a script cannot: in an email, on a billboard, in a printed leaflet, on somebody else's site.
Creating one
Create links on the company's Links screen, or over the API with a key holding
links.write. Each carries its destination and its campaign, and the
campaign is what ties the click to the spend.
QR codes
Every link has a QR code, which is the same link and therefore the same measurement. A scan from a leaflet lands in the funnel next to a click from a search ad, and the report does not have to treat them differently.
Use one link per placement rather than one per campaign. Two placements sharing a link cannot be told apart afterwards, and no amount of reporting recovers the difference.