I know how important it is to have a marginal VAT solution in QuickBooks Online, @client-mazeaccou.
Currently, the margin scheme is not supported in QBO. You need to compute your return outside of QBO. You can utilize the QBO figures to assist you and then submit the data via a spreadsheet.
Hence, there are third-party apps available that can help you manage VAT. You can go to your Apps menu in QuickBooks or use this link to find one: Apps help QuickBooks run better.
It's important to consider consulting with an accountant to help manage these transactions. If you don't currently have an accountant, you can check this page for finding accountants in your area: Find an Accountant.
Moreover, here are some links you can read more about VAT:
Don't hesitate to comment below if you have questions about managing Value Added Tax in QuickBooks Online. I'm always here to help. Have a great day.