AliW

I have a job un which I pay PAYE. I have a small business which I am looking to register this year as I expect to earn more than £1000 for the first time. As part of this business, I will share classes on a US platform which currently witholds tax on monthly payouts eg, 1 class may earn $185.80 but the company withold $26.90 tax. Do I have to declare these payments as income on a self certificate and would I have to also pay UK tax? I don't understand how all this works. Thank you