Welcome to QuickBooks, CDVS. I see this is your first post, and I want to say we’re thrilled to have you here. Getting started with something new can be a bit challenging, but you’re doing a great job so far. Let’s dive into some browser troubleshooting to make sure subcontractors are added seamlessly.
It's correct that subcontractors are added as suppliers, in which you'll then tick the "This supplier is a CIS subcontractor" box to enter their CIS details. This box is located under the Additional info section, as shown below:

Since this option isn’t visible on your end despite enabling CIS, let's consider clearing your browser’s cache. Corrupted data causes unexpected behaviors, including missing features and errors.
If the issue persists, you can access your company using a different supported and updated browser to bypass browser-specific problems and ensure everything works as intended.
Our goal in the Community is to make your QuickBooks Online experience as smooth as possible. If you need any assistance with subcontractors or anything else, don't hesitate to leave us a comment—my team and I are always here to help you every step of the way. Wishing you a wonderful day ahead!