How can we get the Amplitude ID on the front end?

Here is the situation:

• We want to pass back a unique user identifier on our sales form submissions to improve matching with Salesforce data (currently match on email, but emails get changed)
• Previously used the user_id provided by the analytics platform which could be pulled from a first party cookie
• Now we want to use the Amplitude ID, but have no way to surface on the browser when the form is being submitted

Is this possible to achieve? 

We tried combining session_id (amplitude.getSessionId()) and device_id (amplitude.getDeviceId()), but the values are not unique.

Is there anything in the AMP_******* first party cookie we could use?

Using the GTM Amplitude template on our webpage if it helps

Thank you!