How to set require the field to a company in billing address in magento2?
You should as well add it in your attribute on the server side.
If you're using Magento Entreprise Edition, you can simply edit the company attribute through back end, and set it to "required".
If you're working with a Community Edition, you'll have to manually change this value with SQL. It's in eav_attribute table, the attribute_code is company and you just need to set is_required to 1.