Additional input field in order form

Greetings!

Our client wants an additional (not obligatory) input field in the order form. Client’s customers would insert their company’s internal order number for their evidence. The number should then be written on the Magento’s invoice that is sent to the customer. The filed should be on the order page, where customers put their name, address, VAT, etc.

Are there any good existing modules that would do the job? Or is there another way to insert the input field?

I’m quite new to the Magento 2 so any help would be much appreciated.

Thanks!