The method Tom mentioned will work for that situation.
An alternative approach is to create a "passthrough" page that is hosted on Marketo. You can effectively track any lead that is cookied by Marketo using this method by setting up a triggered "visits page" campaign. You might need some dev resources to accomplish this.
Here's an example:
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User clicks link on external landing page that doesn't have munchkin code
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Link directs to a Marketo landing page that has an instant redirect to the actual page the user is supposed to see
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User doesn't see the "passthrough page" and lands on the actual page, which doesn't have the munchkin code
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A "visits web page" triggered campaign is set up to look at the users who land on the page in Step 2
Note that Step 2 needs a redirect which can be accomplished using several methods. The most "white hat" way (strongly recommend this) would be to use a "htaccess" redirect.
Here's an article about that method: https://kb.mediatemple.net/questions/242/How+do+I+redirect+my+site+using+a+.htaccess+file%3F