Hi Ney Bermudes,
If you are having a managing role as (admin), then you can create a user from the account overview page which is under the Administration> User management >Admin >Account overview page => Here you can create a user and login with that user credential.
If you are don’t have an account then you need to create a registration page.
Note: In the Domain model you need an association(1-1) from your entity to account entity .
In the Save button, you need to call a microflow to map the user with an account.