How to Add a external subdomain if it only points to my server IP address

A main domain created a subdomain that points to my server IP address. I have no access to configuring the main domain, nor the subdomain. But i have access to the whole server where my site is hosted.
Is it possible to add Cloudflare under those conditions?. If yes, How can i do it?.

Summary:
maindomain.com has
subdomain.maindomain.com —> IP Addres of my server.

I have access only the de config of my server, not to the maindomain nor the subdomain.

Thanks in advance for any help.

Is any of it currently set up on Cloudflare?

Hi, Im not sure if i understand the question.
If you mean a page from my hosting server, yes, i have 2.
If you mean that the maindomain.com has already something. no.
The main domain is: cinvestav.mx the subdomain pointing to my hosting server ip address is ryma.cinvestav.mx
But when i introduce the main domain here (trying to add it), i see that they have their own DNS.
So, it is not in Cloudflare.

What do you suggest?.

You can only use Cloudflare if cinvestav.mx uses Cloudflare DNS.

Cinvestav looks like a school that uses their own DNS, so I think the answer is “no.” You won’t be able to use Cloudflare for your subdomain.

=(
Too bad. Thanks a lot.