Supply those certifications from various engagements. If through Upwork, you can download certificate of work through the platform. Or, ask 2-3 of your clients to create (or you can create for them) and have them sign, some cert you work with them, how long, how much etc or simply, invoices.
They might ask you for business registration or stickers of incorporation, so I would recommend you include it, to prove you have access to those funds.