Get my external users to pull up a site and access a link on a form to upload documents.

You can set the access rights of a new form or your existing form that includes the document upload field or URL field visible to your external users, and share the link of this form. You also have the option to embed the form on your website.

For your external users to be able to see a URL link you would like to send them to whenever they access the form, you might want to make a static text field, which you can set to an external link. This would show up on the form your external users see as a link.