Solved! Go to Solution.
The above link to the Marketo resource page doesn't work. Correct link is: Adding custom validation to a Marketo form before submitting it
But be warned, the information in that help article appears to be incorrect as well.
And I agree with the original post - having a Confirm Email Address functionality is pretty standard stuff, and should work out of the box in Marketo.
Marketo already have a script in place that you can use and modity. I have used their script with a slight modification - happy to share the script that I added in my blog here, along with a step-by-step tut on how you need to implement it: https://www.campaigntrackly.com/marketo-add-email-address-confirmation-to-your-forms-in-5-easy-steps...
Hope this is useful!