How to create HttpClient with certificate?

Question

HttpClient is created in program.cs with this code:

using System.Security.Cryptography.X509Certificates;

internal class Program
{
    public const string LhvClient = "lhvclient";
    static void Main(string[] args)
    {
        WebApplicationBuilder builder = WebApplication.CreateBuilder(args);
  
        const string lhvprivaatvoti = @"-----BEGIN RSA PRIVATE KEY-----
    MIIEpAIBAAKCAQEAtlfiLYZ48IwU3cbJA69m5F9xCiamWzEvkzseP8aAuLHDVdUi
    AVIzvKZeILr/M9Sc+wowLYz0azBV64YiFI9qoder4BcfkfvznjRKwPUl6wQhOMUG
    hkM79VusdcUAiP31f0pVTtEKhmMqJik5asalB1TFdjylaW3VbrkHqdXVFw91Q3aK
    ifrc4Z6KDPt6wqDCXe108mG0QyaSGtc0lBuV0ZoWL3ZNHrbh3NRV4Ojb/O3mAog8
    rV0FbAufy0qZnaTse52x8KqZDE2rJjNWr23QLk2OAezt6t7a2ZCOLF3OS+tMqyeu
    M8N1lqQIORFo18iYxAXt5Rd9g6Ap6KpE8ahzuQIDAQABAoIBABLtFPJ+u6d+vV0L
    mKpY7AgWM/McHIuTPl88yqCkp5BZZcSkm6P2y/HvyWNXpXd+wPskN+idHHLSZd34
    DSovJ0qRyoA28OBD038p6acU4Duq27vlug342oysAumHv3d4ofZKrodra86Tv7iN
    nNe4CJcLNc0A8cul9ST0muwGNYOaup0wOqLOskfvXmhqYT8gu9h8sbsW40WupIip
    OFUwlGXBwtWCycJTD/UdsCKQZZ1oTX/oWpXDslzUyeUwssgaFdJwAw2Y1IQDODLk
    oV9pSjr2inWfIz0ILbtm131zSTY7BSMspbW1IKT1WJlrJz5gAYJjOddsoZ8RwUtX
    OHnPBFUCgYEA3InyQPWue5MU0j2PSNY3WQaMg/jD1zelmWZ1M65mxihfl/Ctf33I
    8mtEGoWzwaNgRcyTgd2SqOxHMUqeJ1b6mVSRgnC8GtSV32AG/k3qpZ6BB/rLZu5E
    hOcCoqysdF/ng4y1blVoZeJ4u06VI2tSjrp79M2aHh5WNEyEuZ4xSZsCgYEA06mw
    9AXg1LgwsBQJpNbMRwK7UVoJgjMiwKfsMkIDe2keeJo5eD68cseyOyvRjcssft8d
    Sxi3AAMa0ymHOi7/m013PHkfzVSeT0nmL2iGZ1s+w9EnVzwzkNXrFL9T9/QPTOTS
    75tPcbmWvMxwBxgdE5GouVpI+/PSAXYKoVQPxzsCgYEAyAnkECII2xQFPdISxplv
    6LN0/zvEv9E8xxDVXERj+neihdoMNWktvR2oz3nZG9oKOCWg4pnNLqXqyX4KSFqv
    wiagObXyGVAchVm/3ilknkdQeKF2n+2dfwNfM5j2cDSRdZRK+UJFCK1Mn3Fe+5qc
    btQeHWXk1k7kGFoTxZ4EzOMCgYApA3mZX0Y4kdRFWiygN0rv+5SGZ/bttrDmOeOn
    vWjlUfIolmHKbgWgDBf6JTx+yD6/+sW3VnunUfKxthtQ5+h5lGIlYqcJ53qhjIVA
    7HUWs/SOhwmjerPXPcxGgehoZG9OjAxfh222cKrHvKl8hmyj7RaPi/IWeCFaTSA0
    MJC6HwKBgQDVLOevx/XyYiIkOCNn90kyPBNOhMz7vo8vooE8sHCr7y/NerCZfDxG
    1c74EBD8aAyLDbQWh3aoi698TvVhbwtsLwh+KqkajSuGnVKVkhMFm+H6Pny/UF16
    jMQa/SSC4BBj5Y8IIPXoJC4HNlqocCBKHUgQHAurmXMR4ExHCq8ROg==
    -----END RSA PRIVATE KEY-----";
    
    const string lhvsert = @"-----BEGIN CERTIFICATE-----
    MIIE4TCCAsmgAwIBAgIGAZPGzbosMA0GCSqGSIb3DQEBCwUAMGgxCzAJBgNVBAYT
    AkVFMREwDwYDVQQIDAhIYXJqdW1hYTEUMBIGA1UECgwLQVMgTEhWIFBhbmsxDTAL
    BgNVBAsMBFBTRDIxITAfBgNVBAMMGExIViBQU0QyIEludGVybWVkaWF0ZSBDQTAe
    Fw0yNDEyMTQyMDEzMTFaFw0zNDEyMTIyMDEzMTFaMIGFMR0wGwYDVQRhExRQU0RF
    RS1MSFZURVNULThmMjBmZTEeMBwGA1UEAxMVUFNEMiB0ZXN0IGNlcnRpZmljYXRl
    MRIwEAYDVQQKEwlQU0QyIFRlc3QxEDAOBgNVBAcTB1RhbGxpbm4xETAPBgNVBAgT
    CEhhcmp1bWFhMQswCQYDVQQGEwJFRTCCASIwDQYJKoZIhvcNAQEBBQADggEPADCC
    AQoCggEBALZX4i2GePCMFN3GyQOvZuRfcQomplsxL5M7Hj/GgLixw1XVIgFSM7ym
    XiC6/zPUnPsKMC2M9GswVeuGIhSPaqHXq+AXH5H78540SsD1JesEITjFBoZDO/Vb
    rHXFAIj99X9KVU7RCoZjKiYpOWrGpQdUxXY8pWlt1W65B6nV1RcPdUN2ion63OGe
    igz7esKgwl3tdPJhtEMmkhrXNJQbldGaFi92TR624dzUVeDo2/zt5gKIPK1dBWwL
    n8tKmZ2k7HudsfCqmQxNqyYzVq9t0C5NjgHs7ere2tmQjixdzkvrTKsnrjPDdZak
    CDkRaNfImMQF7eUXfYOgKeiqRPGoc7kCAwEAAaNzMHEwbwYIKwYBBQUHAQMEYzBh
    BgYEAIGYJwIwVzA5MBEGBwQAgZgnAQIMBlBTUF9QSTARBgcEAIGYJwEDDAZQU1Bf
    QUkwEQYHBACBmCcBBAwGUFNQX0lDDBNGaW5hbnRzaW5zcGVrdHNpb29uDAVFRS1G
    STANBgkqhkiG9w0BAQsFAAOCAgEAhYm3S+mdyR837aTYk5ZGyXQ5d9ocSiHFDRnU
    j0AxDRCqJ70/jmj0vur4NfAeOC4yz3YXr2jBl0yPBDaXATUQClbMyYr4QIQJLa0Y
    FJ2iDQtN4j6jvi1GcWAyvlW8XSzi1y7mv6ggvkomhkkwd6Ua1EYqduxfrmbu7xOH
    bPd9QgJJkGCY5JmbNAAEL9W12ErZVR5zbJqieTxnetl6clApE4ZuSXWjhyNCbztk
    UJ/9UEZekMiLUCaxiFHQR81UxH7gMzjawTVL54xszddZZ8Tya5J3ayQ5XwpA6t0C
    u2UfhND2f/V8M5N0PaEaXJq+ekeE/eXx8cf/lFr3kesGWCN395NLAfG+W+Ov7d5b
    GBlnLBaZMcQ70Kv3GNHSFsILllJbv92NK28c00/Gyoo8bekCyMcVKv4rEy9vxLW4
    zUkDJOZJ5cM+w1jqgpfQG52rYHpRr1uY0rZx2R8DnXKp+rZ/m6u0uuEqwi5jTk2+
    zjXGocNBGdh5JsDlw31n/SmgzCn4KLH+RPHKCRtBB20oZBcy0ApVvMRCA45T5NKY
    K1Pavic5YCgg2jM2APN6xH6SCdmzJhipOXmYOwYVYW5QYIl8du52xIhEGSzzCiEJ
    zho4eB9Cgti+BJTdYOq0AV3Ad2EUckEkXhVG5u4N5MdhDHqk2gC3IsEpMAwb8J3b
    sT3qlI0=
    -----END CERTIFICATE-----";
    
    var lhvhandler = new HttpClientHandler
        {
          ClientCertificateOptions = ClientCertificateOption.Manual,
          ServerCertificateCustomValidationCallback = 
             HttpClientHandler.DangerousAcceptAnyServerCertificateValidator
        };

    var sert1 = X509Certificate2.CreateFromPem(lhvsert, lhvprivaatvoti);
    var sert = sert1.Export(X509ContentType.Pfx);

    // laaditud value is null. How to load sert?
    var laaditud = X509CertificateLoader.LoadCertificate(sert);
    lhvhandler.ClientCertificates.Add(laaditud);

    var lhv = builder.Services.AddHttpClient(LhvClient);

    lhv.ConfigurePrimaryHttpMessageHandler(() => lhvhandler);
}

In this code, X509CertificateLoader.LoadCertificate(sert) returns null.

How to fix this so that my HttpClient can created?

This is an ASP.NET Core 9 MVC application.