How to use x-www-form-urlencoded in rails

If you're working with a Net::HTTP object and therefore can't use the post_form class method, encode the form values yourself, and provide the encoded value as the data string.

def post_form(path, form_params)
  encoded_form = URI.encode_www_form(form_params)
  headers = { content_type: "application/x-www-form-urlencoded" }
  http_client.request_post(path, encoded_form, headers)
end

def http_client
  http_client = Net::HTTP.new(@host, @port)
  http_client.read_timeout = @read_timeout
  http_client.use_ssl = true
  http_client
end

This is what Net::HTTP.post_form does internally.

To put it in simple terms, if you need to POST a application/www-url-form-encoded request:

# prepare the data:
params = [ [ "param1", "value1" ], [ "param2", "value2" ], [ "param3", "value3" ] ]

uri = ( "your_url_goes_here" )

# make your request:
response = Net::HTTP.post_form( uri, params )
if( response.is_a?( Net::HTTPSuccess ) )
    # your request was successful
    puts "The Response -> #{response.body}"
else
    # your request failed
    puts "Didn't succeed :("
end

Request bodies are defined by a form’s markup. In the form tag there is an attribute called enctype, this attribute tells the browser how to encode the form data. There are several different values this attribute can have. The default is application/x-www-form-urlencoded, which tells the browser to encode all of the values.

so when we want to send data to submit the form by those data as a params of the form the header will send application/x-www-form-urlencoded for define enctype

http.set_form_data(param_hash)

For your

params =  {
         :code => "#{code}",
         :redirect_uri => '/auth/exact/callback',
         :grant_type   => "authorization_code",
         :client_id   => "{CLIENT_ID}",
         :client_secret => "CLIENT_SECRET"
       }
  uri = URI('https://start.exactonline.nl/api/oauth2/token')
  #

  Net::HTTP::Get.new(uri.request_uri).set_form_data(params)

or for post request of form submission use Net::HTTP::Post

and encode_www_form is:

It Generate URL-encoded form data from given enum.

URI.encode_www_form([["name", "ruby"], ["language", "en"]])
#=> "name=ruby&language=en"

in your case

uri.query = URI.encode_www_form(params)
#=> "code=aas22&redirect_uri=...."

More info Here