Darren,
I am not sure what your question means. However, if what you are asking is how to capture a value from the URL and into a hidden field. You just need to make sure that you set the configuration of the field to the value you intend to use on the URL.
For example, if your hiden field's name is lead_source, but you want to use the key/value pair ls=value, just set it up in the field's configuration.
I hope that helps.
Godd luck,
Rafael