Your form method is set to GET. In this case, when Submit is clicked, the client/browser sends request with values in URI:
GET /index.html?data=user_value&default_name_of_button=Submit HTTP/1.1
...more lines to follow...
These HTTP header lines will be available on your EthernetClient instance, retrieve them line-by-line using connected(), available(), read() methods. Then parse the line, find the user_value.
Please note that browser retrieves your page (to display it) using GET method too, in that case URI will be like:
GET /index.html HTTP/1.1
...more lines to follow...
It is recommended to use POST method to submit form values. Example content section (before content, server must send HTTP header - check Arduino Web Server example)
<form method="POST" action="foo.php">
First Name: <input type="text" name="first_name" /> <br />
Last Name: <input type="text" name="last_name" /> <br />
<input type="submit" name="action" value="Submit" />
</form>
In this case the values are returned in content section:
POST /foo.php HTTP/1.1
Host: localhost
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Referer: http://localhost/test.php
Content-Type: application/x-www-form-urlencoded
Content-Length: 43
first_name=John&last_name=Doe&action=Submit
Tutorial on HTTP headers:
https://code.tutsplus.com/tutorials/http-headers-for-dummies--net-8039
