I'll provide an answer to your question, Entmas.  

Before diving in, let me clarify one thing. Are you referring to EIN or State tax registration? If so, I'll show you how to enter it. To access the Payroll Settings, simply follow these steps: 

1. Click the Gear icon. 
2. Under Your Company, click Payroll Settings. 
3. Click Federal Taxes and enter your EIN, then click OK. 
4. Tap State Taxes - (your state) and enter your state account number. 
5. Press OK. 

For your reference, you can go through these articles for more detailed information. 

• What is a state EIN or account number? 
• Prepare and file 1099s 

Feel free to leave a reply below if you have other concerns with QuickBooks. I'll appreciate the opportunity to help. Stay safe and take care always.