Getting url request parameter value within the SAIL form

We have a requirement to get the request parameter passed in the Appian sites url from an external non-Appian application and use the request parameter value inside the SAIL form.
We are planning to write a custom expressions function plugin, so we can use the function to retrieve the parameter value inside the SAIL form. Can someone please advice from which class file can we get the request object value or any other options?
Eg: When we click on this link from external application it goes to the Item Summary site but there is no option to get the itemId value - https://test.appiancloud.com/suite/sites/item-summary?itemId=value

We also tried to implement using custom servlet plugin to get the value from the url but there is no option to again pass this value to the sites page.