Quick guide on getting a GUID from the URL in CRM 2013: (they look as though they have removed them in CRM 2013, but they are just hidden!)

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Open the form you want the GUID of and click the ‘pop out’ icon found in the top right corner:

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This will open your record in what looks like a new window, but now if you look at the URL:

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You can see the GUID nested nicely in between curly braces. If you just want a sanity check, click the ’email a link’ icon to get the reference in a link, and you can cross check the GUID against the GUID in the email a link, starting after the small ‘b’ ending at the one before the ‘%’.:

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You can see they both match!