To set up your VAT ID, all you need to do is click on "Company" in the sidebar, select VAT  and "Add VAT ID".

You can also setup VAT ID by clicking your name at top right, choose "Profile". Go to "Companies", select the "Company", choose "VAT" and " ADD VAT ID".

Kindly do note: 

Your company's name must match the company name exactly to pass the verification. Use this tool to see how your company's name should be: