An option is available to automatically connect the user to a record when submitting the child form. To configure this feature:
Make sure that login is enabled in the portal
Go to the Portal Tables section
Open the child form of one of the tables
In the Fields section, make sure that the linked record field that is linked to the User's table is available.
Click on the linked record field.
Go to the Advanced section
Turn on the Add-only mode toggle
Auto-fill the field with logged-in user record toggle will appear
Turn on the Auto-fill the field with logged-in user record toggle
Additionally, it's important to configure the custom view to display all records in order to make full use of this functionality.