Hello spobjecky,
Welcome to the Community. I'm glad to offer my assistance so you can get those estimated tax payments entered.
Schedule C Estimated Tax payments are recorded in QuickBooks Self-Employed three ways. Here's an article that explains each way the entries are made: https://community.intuit.com/articles/1629827-schedule-c-estimated-taxes.
My suggestion would be to record these tax payments as Expenses and categorize them accordingly. I'd consult your accountant to be sure.
This link will take you to the QuickBooks Self-Employed email window if you'd like to relay your question to an agent: https://help.quickbooks.intuit.com/en_US/contact.
I hope my response helps. Please let me know if I can lend a hand with anything else.
