certreq.cs

using System;
using System.IO;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;

using var ecdsa = ECDsa.Create(ECCurve.NamedCurves.nistP256);
var req = new CertificateRequest("CN=Kevin Jones", ecdsa, HashAlgorithmName.SHA256);
req.CertificateExtensions.Add(new X509KeyUsageExtension(X509KeyUsageFlags.DigitalSignature, true));
req.CertificateExtensions.Add(
    new X509EnhancedKeyUsageExtension(new OidCollection {
        new Oid("1.3.6.1.5.5.7.3.3")
    }, false)
);

var reqDer = req.CreateSigningRequest();
File.WriteAllText("reqest.pem", new string(PemEncoding.Write("CERTIFICATE REQUEST", reqDer)));
File.WriteAllText("key.pem", new string(PemEncoding.Write("PRIVATE KEY", ecdsa.ExportPkcs8PrivateKey())));