How does one identify teh correct variable names when writing HTML code for web applets?

I am trying to create a web applet that pulls all Custom Object 2s (CO2) that are related to a given CO1. The code is below. The variable name I am using for the p3 variable does not work, so I assume it is incorrect. Right now, I am just trying different names for the CO1 Row Id, in other words, I am using trial-and-error in the hope of finding the correct variable name. My question is: how can I identify the correct variable name without resorting to trial-and-error? Thanks for any help you can give, Martin Cryer

<iframe src='/OnDemand/user/analytics/saw.dll?Go&Path=%2fshared%2fCompany_AKWA-XO8X5_Shared_Folder%2fReports+for+Generating+Unit+Detail+Web+Applets%2fTurbines&Action=Navigate&p0=1&p1=eq&p2="Custom Object 1".ROW_ID&p3=%%%RowId%%%&Options=r'frameborder="no"WIDTH="100%"HEIGHT=100></iframe>