I see, so this button is in a form?

Have you considered adding properties as hidden inputs?

I’m not sure why this was added to rails, but conceptionally it seems quite strange to exist. GET parameters are part of the url, so I’m not sure why they should be added via options on the button. Phoenixs’ route helpers allow you to add GET parameters to urls as well as the URI module of the elixir core. If you’re looking for additional body params you’d need to wrap the button in a form to submit, which has some hidden inputs.

Phoenix’s button handling with method is mostly a convenience to not need to wrap just a button with a form, to defines nothing more than a url/method. If you need more data as part of the request use a form with hidden fields. You’re imho past the point there the convenience does make much sense.
For get requests you could alter the url, but a form will also just use get params if you submit it with the GET method. I guess both ways are viable for get parameters.

If you’d use form_for instead of content_tag(:form, …) you’d get the csrf token and the method override automatically and it would therefore work for any http method.