I know nothing about siding. But I do know that window boxes are very heavy. They need to be attached to very strong timbers or masonry. Unless you have window sills they can sit on. Or, if they're on the ground floor, they could have legs.

Window boxes do not get hung from siding. They get attached to the structure of the house. Tando is a vinyl siding and vinyl move a lot. I would personally not attach windows boxes thru the siding as you very well may trap the siding and cause it to buckle without enough expansion and contraction room. Not to mention they are a PITA to water and maintain and next year you will be asking how to remove the stains from the siding.

Millworkman, thank you. I agree about not drilling through the siding. However, what can I use as an alternative? The boxes are PVC and mainly filled with styrofoam and just enough soil for the plants to grow. They are very easy to reach for watering and are currently attached via brackets drilled into the wood.

" However, what can I use as an alternative? "

You either use a wide trim around the window and attach thru that or you do not use the window boxes.

The window box hanger installation should be done by your siding installer. It can't be that big of a deal.

Unless there are substantial timbers or masonry beneath the siding directly where the boxes are required I don't know how this will work. Even plastic boxes would be heavy if they contain sufficient damp soil to successfully grow plants. I can barely lift mine.