Things like this never work.
People aren’t bothered enough about window cleaning to join. Most people will ask a friend/neighbour or google a local window cleaner.
In fact here’s a thought, I’ve got a decent website with a fair bit of content on it, and not once has anyone ever seemingly read any of it. They just find the website and ring me as soon as as they find the contact details.
I know this because they always ask questions that are answered clearly on my website. People don’t read it , they find me and ring.
So if that’s the case with joe public, they’re not gonna bother joining some rewards scheme.