I'm here to guide you in adding a customer, midpins.
In QuickBooks Self-Employed, we can only enter their details manually when creating an invoice. Simply click on Invoices tab, then select Create invoice. At the upper part of the invoice, enter your client information.
This version of QuickBooks is designed to track only the income and expenses of your self-employment. Though, I'll send a feedback to our developers to have an option to save customer details.
Also, please visit our Help page if you have more questions about QuickBooks Self-Employed. You can also reply below anytime. I'll be here if you need more help.