Amplitude tag how to identify unique users using user id

[jasonblood]

We are implementing the Tealium tag on our apps. In the past we previously used the Amplitude SDK and we are able to identify users in tealium via a user id. More explained in there docs here. 

https://amplitude.zendesk.com/hc/en-us/articles/115002278527#setting-custom-user-ids

We want to use tealium to send data to amplitude instead of including the SDK in the app. But I cannot figure out how to set custom user id in tealium. We have a user id setup in our datalayer. But I don't know when setting up the mapping the in the amplitude tag which destination I should use for Amplitude. E.g I don't see any user id related destinations. Is this possible as not being able to identify users limits the usefulness of the amplitude tag for our use case.

[michaelbenefiel]

Hello,

I set up an instance of Amplitude in my local environment, and I'm showing "userId" in the network call after I map my UDO user id (testUserId) to set.UserId (screenshots below). While I'm unable to verify if this is making it to Amplitude, it'd be a quick way to test. I've also seen recommendations to set the userId via tag-scoped custom JavaScript extension, much like the one below. 

var count = 20;
var interval = 250;

var setUserId = setInterval(function() {
    if (count > 0) {
        if (window.amplitude && b.customer_email) {
            amplitude.setUserId(b.customer_email);
            clearInterval(setUserId);
        }
        count--;
    } else {
         clearInterval(setUserId);
    }
}, interval);