If you are the owner of andreivalentin.com domain which is using Cloudflare, you can easily add a new A training or CNAME training type DNS record (hostname) on the DNS tab of the Cloudflare dashboard - beased on the needs.
Kindly, find below some usefull articles and helpfull tutorials.
To create a new sub-domain, navigate to the DNS tab first.
Therefore, you click on the blue button “+ Add record”.
After that, it should ask to specify whether it needs to be an A record or CNAME, or any other.
Provide the name of the subdomain training and point it to your host/server IP address (put the value into the “content field” of that new record).
Click on the blue button “Save” to apply the changes.
Make sure that new record is cloud (proxied via Cloudflare).
More about the DNS tab:
About adding a DNS record(s):
Editting an DNS record(s):
Regarding a redirection, you could achieve this either via Cloudflare Page Rules from below articles: