ServerCertificateValidationCallback (or rather, collector) for .NET Core

Get-WebCertificate.ps1

[CmdletBinding()]
param(
[Parameter(Mandatory)]
[Uri]$url
)

Add-Type -TypeDefinition @'
using System;
using System.Net.Http;
using System.Net.Security;
using System.Security.Cryptography.X509Certificates;

public static class CertHelper {
    static X509Certificate2 LastCert;

    #nullable enable
    public static bool Validate(HttpRequestMessage requestMessage, X509Certificate2? certificate, X509Chain? chain, SslPolicyErrors sslErrors) {
        if (certificate == null) {
            return false;
        }
        LastCert = new X509Certificate2(certificate);
        return true;
    }

    public static X509Certificate2 GetCertificate(Uri url) {
        var handler = new HttpClientHandler();
        handler.ServerCertificateCustomValidationCallback = Validate;

        var client = new HttpClient(handler);

        HttpResponseMessage response = client.GetAsync(url).Result;
        response.EnsureSuccessStatusCode();
        return LastCert;
    }
}
'@


$certificate = [CertHelper]::GetCertificate($url)