Existing user login from external database or non-Bubble website

Hey all,

We are currently working on building an app for a client.

The client has requested users of their current website, be able to Sign In into the new Bubble app from their website or vice versa. This is because they want their current user base to access the new app without having to create a new profile.

We have success signing in users with two Bubble apps in the past, but not with a non-Bubble platform.

Anyone have success using existing user login details from external databases? Any pointers would be great!