Specify Subject Alternative Name when generating a self signed certificate

Is there a way to get a .crt and .key file with the subject alternative name set? I am configuring a proxy with an openssl .crt and .key generated by this command

openssl req -x509 -sha256 -nodes -days 365 -newkey rsa:2048 -keyout proxy.key -out proxy.crt

I then cat the .key and .crt to get a .pem and use that on the client side.This cert works fine for securing the https connection but I get a warning that the Subject Alternative Name is not set in the certificate. In another client I use the warning is actually an error that terminates the connection.

The solution here https://security.stackexchange.com/a/91556 gives me a .csr which I rename to become the .crt I need, and when I use this with the client the https connection fails on incorrect ssl certificate.