The method to achive that goal is very invasive and may be illegal in some jurisdictions.
It can be achieved using JavaScript onfocus and onbluer events.
The onfocus event occurs when an element gets focus.
Onfocus is most often used with <input>, <select>, and <a>.
object.onfocus=function(){SomeJavaScriptCode}
The onblur event occurs when an object loses focus.
Onblur is most often used with form validation code (e.g. when the user leaves a form field).
object.onblur=function(){SomeJavaScriptCode};
You can access the value of the element like this:
name = myForm.elements["name"].value;
The idea is capturing the field value when the focus moved to or from a field and then silently submit through SOAP API syncLead along the cookie id.
http://developers.marketo.com/documentation/soap/synclead/
Please keep in mind SOAP API will set the lead to known even though it may not have any identifiable elements.
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