How do I import by certificate authority

I feel dumb...

I have a private CA, I have a private key and cert for OPNsense. I want OPNsnese to acknowledge the infinite wisdom of that CA, and use the key and cert I have bequeathed unto it. I cannot seem to figure out how to make it do that.

I add the cert and key in Certificates, but to make it into a Server Certificate the UI insists I must provide a CA key...and if I try to add my CA's cert as a CA, it also says I must provide a CA key...and I don't know what that even means in this case, because the whole point is that the user of a CA shouldn't know the key...

What am I missing? Thanks.