It seem PLC S7-1200 use form authentication, which require preserve cookie between login url and protection url. The arduino by alone is hard to preserve cookie if not impossible.
The cookie is sent in the HTTP header, for example:
HTTP/1.1 200 OK
Date: Fri, 28 Apr 2006 03:14:58 GMT
Server: Apache/2.0.46 (Unix) PHP/4.3.2
Content-Type: text/html; charset=iso-8859-1
That sets two cookies (foo and food).
When you receive the HTTP header you could save those.
Then when you reply you also have a Set-Cookie line and return the cookie value.