There are lots of devices like this - birds deposit something and receive a reward. All shapes and sizes, different techniques for detecting a deposit and providing a reward. During the design and build, the dispenser assembly was the part I spent the most time on.  Once you have one ready to go, it's all about getting the birds to do what you want.

This approach worked for me with pigeons and jackdaws. Durations here are based on my experience with jackdaws.

0 - Small amount of food in the area you will mount the vending machine to attract birds (~1 week)
1 - Food Provided, Timed Dispense, Deposit Provided (1-2 weeks)
2 - No Food Provided, Timed Dispense, Deposit Provided (2-3 weeks)
3 - No Food Provided, No Timed Dispense, Deposit Provided on the Platform (3-4 weeks)
4 - No Food Provided, No Timed Dispense, Deposit Provided Near The Platform (TBD)
5 - No Food Provided, No Timed Dispense, No Deposit Provided (GOAL!)

Pigeons got to Stage 3 in less than two months but didn't show much promise to progress beyond that. Jackdaws got to Stage 4 and may make it to Stage 5 under the right conditions.

Here are what I think are key aspects to training pigeons and corvids to deposit stuff for food.

Encourage Accidental Deposits

Hans Forsberg used this approach to train his magpies. A key thing to do is to mix the reward and the items you want birds to deposit on the platform. They will toss around and kick non-food items, often accidentally depositing one. After doing this 10 or 20 times they start to associate depositing those items with food. I found that they are likely to ignore other items in favor of the ones you trained them with. I trained the jackdaws with bottle caps and cigarette butts, and they go for that first before considering any other items like screws or scraps of plastic.

Gradually Move Deposit Material Away

There is a big gap between depositing stuff left ont he platform and bringing their own stuff from elsewhere. To get the birds to pick things up off the ground (or floor) it helps to start putting deposit material near the platform. I used a scrap of wood with a dish on the end to provide a path from the platform to the dish of bottle caps. I gradually swiveled this away from the platform over the next week or two so the birds got used to having to bring things to the platform. Then I lowered it over a few more days and finally started putting things on the floor. They then seemed to at least understand that any cigarette butt or bottle cap on my balcony was a potential peanut.

Ground Level

While being above ground does not make it harder to get started, in my estimation it does make it more challenging to get to Stage 5. My balcony is far from the litter on the ground. I think they view it as a box they can visit to get a peanut if there are caps or butts around. In a more open, ground level area I think they would not think of these artificial boundaries and it would be easier to train them to bring things from farther away. This one is just my hypothesis - I haven't lived at ground level with any outside space for a while.