In the OnWeb Developers Guide there is an example like this on page 44:

Using Host Publishing with OnWeb ActiveX control
You can access the Host Publishing engine through the OnWeb ActiveX
control’s rule called “_Update”. The parameter for the _Update rule
represents the entire CGI stream containing the submitted Host Publishing
HTML form. It returns all the tables created during at run time.

Example

OnwebCtrl1.ClearParameters
OnwebCtrl1.AddParameter cgiFromIIS
OnwebCtrl1.RunARule("myHPApp._Update")

What is the name of that parameter (The name you set when you create the rule)

The issue here is that in Classic OnWeb (older versions of OnWeb) parameters were specified by sequence and parameter name was not used and that's why you have only one parameter. In Multi Platform (current versions of OnWeb) we use the parameter name and parameter value instead of sequence.

What you need to do is split the cgi and add the parameters one by one to the rule.

Example:

in Classic:  param1=1¶m2=2¶m3=3
addParameter cgi

in MP:

addParameter(param1, 1)
addParameter(param2,2)
addParametrer(param3,3)