I'd do more than just a click link. Sometimes people accidently click without meaning to unsubscribe. We have a bit more complex process, and about 25% of the people who click the unsubscribe linke in an email don't actually complete the process.
Step 1: Create a Marketo Landing Page
Create your page as a Marketo landing page. Use this page as your email unsubscribe link target. Create a one-field form with Email Address Only. Add the form to your landing page.
Step 2: Create your alert message.
Create an email message something like this:
The following person has unsubscribed: Name: {{lead.First Name}} {{lead.Last Name}} Email Address: {{lead.Email Address}}
Step 3: Set up the campaign to process:
Trigger Smart Campaign
SmartList: Fills out form (using name of the form above)
Flow: 1. Send Alert (email you created in Step 2 above) 2. Change Data Field --Field name: Unsubscribed --New Value: true 3. Schedule: Activate
This will set the Unsubscribe field in Marketo, so you won't accidently send a message, and send an internal alert. It also gives the recipient one last chance to keep receiving your emails.
You won't see the link in the dropdown until it has been clicked by some lead at least once. Also, I agree with Steve, it is really unsafe to rely on click like for email unsubscribe.
Particularly because a lead may not have Javascript enabled and the click link event wouldn't be tracked in that case.