Password field on MKTO form - example Javascript?

Has anyone written any javascript for a Marketo landing page that will change a registration form field type to a Password field.  When a person types information into the field, the characters are hidden.

I was wondering if anyone has any sample code that they would be willing to share.

Our use case:
- Register for an account to access our knowledgebase using a Marketo form.
- Have a password field on the form that allows the user to create their password for the knowledgebase
- The form should be a password field so that any characters are hidden when they are typed into the field.
- Marketo smart campaign will immediately delete the password data from the database.

Thanks in advance for any advice.