What you can do, is to build an URL that encodes your diagram text.
Then send this URL so the plantuml.com server using a standard GET HTTP request, and retrieve the HTTP answer with the image.
Building an URL is not very difficult, for example, you have an snippet in PHP here: http://plantuml.sourceforge.net/codephp.html
In Java, you can using this: http://plantuml.sourceforge.net/api.html
Which development language are you using ?