I think he means on a site he does not "control". You need to look at the form structure and you generally should be able to pass the fields as post key/value pairs in the url.
For example, if the form on site http://presidents.com was calling /processForm.cgi in the form tag and had fields fName and lName, you could try calling it like:
http://foo.com/processForm.cgi?fName=Abraham&lName=Lincoln&Submit=1
With that said, any script I personally ever create that takes post arguments checks the referer, so you'd see the message:
Some fool at $ENV{REFERER} is calling my forms remotely!
For example, if the form on site http://presidents.com was calling /processForm.cgi in the form tag and had fields fName and lName, you could try calling it like:
http://foo.com/processForm.cgi?fName=Abraham&lName=Lincoln&Submit=1
With that said, any script I personally ever create that takes post arguments checks the referer, so you'd see the message:
Some fool at $ENV{REFERER} is calling my forms remotely!