Use the following OpenSSL commands to create a certificate signing request.
Create a RSA or EC private key. Keep this file safe and secure!
openssl genrsa -out private_key.pem 2048
or
openssl ecparam -name secp256k1 -genkey -noout -out private_key.pem
Create the certificate signing request.
openssl req -new -key private_key.pem -out csr.txt
Copy the content of csr.txt into the Certificate Signing Request text field. When you've obtained your certificate via an e-mail, you no longer need the csr.txt file.