Hello,
There is a way to pull a member id on the banking core. However, it is slightly different for every Financial Institution(FI). Sometimes it is customer_id, sometimes it is primary cif, sometimes it is a custom field on user data. There is a lot of ways this is identified for various FIs. In general, we recommend to not tie to the core at all whenever possible whether you are an FI or a third party developer. If you really need data from the core, please make a support case on q2developer.com and we can walk you through how to go about this.
If you are just looking for a unique identifier for a user. I would recommend using a combination user_id and either the aba or a identifier you set in a wedge_address_config.
To get this on the frontend. You would most likely need to make a call to the backend to request this data. You can see an example call here: https://docs.q2developer.com/guides/frontend_development/uux_payload.html#making-api-calls-to-your-extension
Hope this helps,
Trey Teter
Software Engineer