Hi LND1212,
If you are self employed as a sub contractor, you are running a business as a subcontractor.
When asked how many businesses your have, you would select 1 and give it a name, it can be just your own name.
This will in turn allow you to enter your turnover (income received) and the tax deducted by the construction industry scheme (CIS).
Please see guidance here:
What you must do as a Construction Industry Scheme (CIS) subcontractor
Thank you.