I recently applied as a freelancer(photo and video editor, photography mentor). I have freelance service contract with well known media agency and also attached my professional portfolio, contract showing company logo, signed and earning income reports, explanation of how I earn money and workflow. The embassy didn’t requested me like that. They requested other things. But in ur case, now that they do, u should ask client confirmation letter with their company letter head and signed. And explain them about your job workflow and how u earn money. (Like how many projects u get per month and how long does it take for u to finish projects). Hope it helps.