ASP.NET ImageButton Click

I am attempting to use SOAPUI to perform a POST to simulate an ImageButton click in an ASP.NET web application. Recording this POST using Fiddler I see that it has the following Hidden Form Elements that are being posted back.

__EVENTARGUMENT=
__VIEWSTATE=%2FwEP...
__PREVIOUSPAGE=-uc_3v....
__EVENTVALIDATION=%2FwEP...
ctl00$MainContent$ddlxxxxx=xxxxxxxx
ctl00$MainContent$ddlyyyy=0
ctl00$MainContent$btnzzzz.x=55
ctl00$MainContent$btnzzzz.y=9

First of all, when I look at the recorded POST, all of these values are submitted even when empty, but in the case of the POST from SoapUI it appears that any field that is not assigned a value is not passed in the POSTed variables in the message body. Is there some way to change this behavior to always pass back all fields whether they are empty or not?

I am also curious if anyone has experience with an ASP.NET Postback based on an ImageButton click, and if there is something aside from the apparent button x-y coordinates that are passed back in the last two POST parameters above.

Thanks for any thoughts.