Hi Eitan,
Ok, that's half of the work. Now, select the hidden field and in Properties scroll down to Autofill. Next to Autofill is Edit link; hit it and new window appears where you need to define:
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Deafult value: if you have default value enter it here if not just type NULL
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Get Value From: this is cruicial part and here you need to select how you will get parameters, so here you need to select URL Parameter from the drop down menu
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Parameter name: enter your URL parameter from link - utm_source
Repeat the same proceedure for all hidden fields. There should be one field for utm_source, another for utm_medium, next for utm_campaign, utm_term or any other parameter you're using.
Hope this helps.
Please note that steps described above are for Form 2.0 editor.