How to identify users and pre-fill fields using the core messaging API

Hi,

On our site, we load the Messenger using the core messaging API and then ask the customer a couple of questions and ask them to fill in their name, email and phone number. At some point in their journey, we already have that data available and would like to pre-fill that for them - similar to the identify call in the Classic webwidget API v1. Is there a way of pre-filling this information for the customer?

I have looked at using conversation fields (see this documentation), however that only seems to allow me to pre-fill custom fields - I might be wrong here. If I am wrong, how can I get the id's of the standard fields?

Another option would be to log the user in using a JWT token (see this documentation). However, in the flow I want to implement this, the user is not logged in and this requires quite a bit more development work, so definitely not ideal

Am I missing something obvious? Could you help me out here?

Thanks!