Movian

showtime.httpReq(url, {
                 'components[0][value]': 'Mozilla/5.0 (Windows NT 6.1; rv:52.0) Gecko/20100101 Firefox/52.0'
});

It does:

user_agent=Mozilla%2F5.0%20%28Windows%20NT%206.1%3B%20rv%3A52.0%29%20Gecko%2F20100101%20Firefox%2F52.0

It should replace spaces to + instead of %20 as described in wiki and corresponding RFCs:

When data that has been entered into HTML forms is submitted, the form field names and values are encoded and sent to the server in an HTTP request message using method GET or POST, or, historically, via email.[2] The encoding used by default is based on an early version of the general URI percent-encoding rules,[3] with a number of modifications such as newline normalization and replacing spaces with + instead of %20. The media type of data encoded this way is application/x-www-form-urlencoded, and it is currently defined (still in a very outdated manner) in the HTML and XForms specifications.