Log in to your Apple Developer account with your main Apple ID, not the new one that you just created, and follow the steps below.
1. Click on Certificates.
2. Click the ‘+’ icon to create a new development certificate.
3. IMPORTANT: Select iOS Distribution(App store or Ad Hoc) & press Continue.
4. Download the CSR file provided here onto your desktop (CSR file)
Select the file after download and press continue.
5. Download the certificate & store it safely, you need to share it with MyStudio dev team.