I would like to try and create a notification task that emails our customers, but the task needs to have a link that will either open on web site if on a desktop. but if on a device with Nintex Mobile App that will open it in the app and the task specifically.
Has anyone been able to get this working yet?
There are a couple of things to consider here. If this task is sent to your customers, do they have logins to your SharePoint environment? If yes, that's great. If no they can't interact other than use lazy approval via an email reply. If you just want the customer to open a generic webpage then you can certainly do this by adding a URL to the page.
Assuming they have access then when you create a task, by default it provides a link to the task itself and the underlying list item. If you've deleted them by mistake, then you can easily add them back:
Unfortunately there is no way that I know of to automatically open this link - most email programs would prevent this from happening due to security reasons.
As for Nintex mobile, what you're asking for is deep linking. This function is not available yet but I understand deep linking is on the roadmap.