It all depends on your requirements. When you simply want a link between the user in Bubble and the tablerows in Backendless(f.i. accessible through an API), a field userid with the unique Bubble Userid in the tables in Backendless will suffice. Add a parameter user id to the API’s, fill it with current users unique ID and you can use it to add and retrieve rows for that user.
If you have additional requirements which require you to register users in Backendless, other options are possible, f.i. check The Perfect Backend And Database For A Bubble App | Backendless
Note: ID’s are normally for internal use only. As an alternative, add an extra field uuid to the user data type on the Bubble side, at creation of a new user, use one of the UUID generator plugin to fill the field and use this for the relation with the data on the Backendless side.