Hi Jamie,
Thanks for the feedback, but I have already checked that link.
And it seems to me the Multipass login feature is available only for shop owners that have Shopify Plus account.
I am not sure did I understand everything correctly, but it seems to me that via Multipass login I can sign in users from my existing website to my Shopify store, but not vice versa.
I am interested to sign in existing Shopify customers to my external website. Of course, for security reasons I can't retrieve current user password, but I was thinking maybe to force user to change his password via Shopify API.
For an example, when a Shopify user tries to sign in for the first time on our portal, I was thinking to send him a link to activate his account on our portal. When he clicks on the activation link to redirect him on change password screen. Afterwards, I can update his password on the Shopify portal as well.
But this is really a mess and I am looking for a cleaner solution, any ideas?