how to prepare soil for dwarf french beans

Haven't you got any compost made? Maybe you could 'borrow' some from a friend or neighbour to dig in! The topsoil that you can buy wouldn't be nutrient-rich. I think you can buy bags of well-rotted manure, can't you? That would do the trick.

At the end of the day, rather than throw them or give them away, stick them in and see what happens. I do this sometimes and surprise, surprise you get results. Maybe not the perfect crop but it's still a crop. Give it a go.
Or try a three to a pot.

It might work to do something along the lines of the soil mixture I made up for the boxes in my patio garden, you could work it into your soil and see if it helps. I used one part peat moss, one part coarse vermiculite and one part MIXED compost. The compost is the most important part for me and I mixed cow manure, hen manure, worm castings and mushroom compost, all commercially available composts here in my area. Mixed together to fill boxes and planters and worked in to the soil might work for you.

I've used bags of well rotted manure bought from DIY/garden centres (there's no smell!) and this has really helped in my garden, although I've not tried it for growing dwarf french beans. Good luck- I hope you get a good crop.