Automatically fill out a form on a website

I am trying to write a script (preferably javascript) that when ran will fill out a form on a different website and submit it.

A good example of what I'm trying to do would be something like this.

Have a script on my website that when accessed can fill in a form like the one below and submit it.
http://messaging.sprintpcs.com/textmessaging/compose

Is something like this possible? Please bear with me as I am a real estate developer that likes to program on the side.

many thanks,
b