First, make sure you have registered as a Digital Platform through your HMRC Government Gateway: 

https://www.gov.uk/guidance/register-to-carry-out-digital-platform-reporting 

Your Platform ID, which is 15 characters long, will be provided when you register as a Digital Platform through HMRC.

Once registered, you can find the correct ID under Platform operators within your HMRC Government Gateway.

Enter your Platform ID in Admin > General Details > DAC7 settings alongside your Legal company name.

This will allow you to then generate your XML report and submit your information.