Sorry… forgot to ask for operating system… 
@patrickse It doesn’t matter, because it was not relevant to the original question. After original question was answered, I got additional problems while using the amazondashbutton binding. I thought maybe some people here could easily help, because of their experience with Dash Buttons. Thanks. 
I got some Dash buttons from eBay, since they are not available in my country (Hungary). Does anyone know if I have to be a Prime member in order to set up the buttons via the Amazon app?
I always get an error when I try. It looks like the phone starts talking to the button, the app asks me for the WiFi network SSID and the key, then it says “Registering”, and after 5 seconds or so, the app says “Registration error”, and the button remains unconfigured.
Does anyone have an idea where I should start looking? Could it be because I’m not a Prime member, or because of the country not being supported, or something completely different?
In every manual I read, they always told: “Login to your Amazon Prime Account”.
I don’t know, but I guess this could be a reason…
I have just figured out what the issue was. The IP configuration on my home automation network has a subnet mask of /23. The Dash button can only handle subnets of a C class or smaller. This sounds like a horrible design.
When I changed the subnet mask to 255.255.255.0 from 255.255.254.0, I was able to complete the setup.