How to update custom fields in project details for Zoho Projects through deluge

How to update custom fields in project details for Zoho Projects through deluge

Hi, I found this resource Project Manager System | Projects API :Zoho Projects but I can't figure out how to update my dashboard details. I have a custom field called "Actual Invoiced" that I would like to update with a custom value through a workflow rule that uses a deluge function. However, I can't seem to be able to update custom fields. Could someone please provide me a code example so I can do the required task.