Add Alternative/Secondary Subject Name to a CSR

create a configuration file based on your needs. you can use the following sample:

# certificate.conf
[req]
default_bits = 2048
prompt = no
default_md = sha512
req_extensions = req_ext
distinguished_name = dn

[ dn ]
C=[country ISO]
ST=[State]
L=[City]
O=[Organisation]
OU=[OrganisationUnit]
emailAddress=[email-address]
CN = [Main Domain Name]

[ req_ext ]
subjectAltName = @alt_names

[ alt_names ]
DNS.1 = [Alternative Name 1]
# or if you have IP address:
# IP.1 = [127.0.0.1]
# IP.2 = [another address]

now you can use the following openssl command to create a CSR based on this configuration

openssl req -new -key server.key -out server.csr -config certificate.conf