You can add your subdomain by following the steps below:
1) Log into your Cloudflare account.
2) From the drop-down menu on the top left, select the domain.
3) Select the DNS icon.
4) Select the record type from the record type drop-down on the left-hand side. A subdomain will either be an A or CNAME record type.
5) Enter the record name and value.
6) Click Add to finish adding the record.
7) Click Add to finish adding the record.