How to write hyperlink custom field in a SharePoint list using GraphAPI

I am trying to update a SharePoint list. More precisely, a hyperlink custom field.

Both Server and URL are custom fields. I can read the URL, then I get:

Description: "http://x.y"
Url: "http://x.y"

I also can write any Server field, as that is a scalar string.

But it seems that the UntypedObject I am constructing is not accepted, and I get Invalid request (without any further details). I have tried both with capitalized and not capitalized names for the UntypedObject keys.

My code is as follows:

var options = new InteractiveBrowserCredentialOptions
{
    TenantId = tenantId,
    ClientId = clientId,
    AuthorityHost = AzureAuthorityHosts.AzurePublicCloud,
    RedirectUri = new Uri("http://localhost")
};

var interactiveCredential = new InteractiveBrowserCredential(options);

var graphClient = new GraphServiceClient(interactiveCredential, scopes);

var items = await graphClient
    .Sites[siteId]
    .Lists[listId]
    .Items
    .GetAsync(requestConfiguration =>
    {
        requestConfiguration.QueryParameters.Expand = new[] { "fields" };
        requestConfiguration.QueryParameters.Top = 100;
    });


foreach (var item in items.Value)
{
    var fields = item.Fields?.AdditionalData;

    if (fields == null) continue;
    
    if (fields.TryGetValue("URL", out var _url) && _url is UntypedObject uo) // testing read
    {
        foreach (var (name, node) in uo.GetValue())
        {
            var scalarAsString = await KiotaJsonSerializer.SerializeAsStringAsync(node);
            $"{name}: {scalarAsString}".Dump();
        }

        continue;
    }

    var table = new UntypedObject(new Dictionary<string, UntypedNode>
    {
        ["url"] = new UntypedString("whateverurl"),
        ["description"] = new UntypedString("whatevertext"),
    });

    var updateFields = new FieldValueSet
    {
        AdditionalData = new Dictionary<string, object>
        {
            ["URL"] = table,
            ["Server"] = "whatever" // this on its own works
        }
    };

    await graphClient
        .Sites[siteId]
        .Lists[listId]
        .Items[item.Id]
        .Fields
        .PatchAsync(updateFields);
}

Request and response:

Seems to be related, but no solution there either: https://github.com/microsoftgraph/msgraph-sdk-go/discussions/687

Started github issue: https://github.com/microsoftgraph/msgraph-sdk-dotnet/issues/2957

Update:

Seems to be not supported ever since, that's shameful to be straight: https://learn.microsoft.com/en-us/answers/questions/645640/how-to-update-sharepoint-list-item-hyperlink-colum

I will try this API instead: https://learn.microsoft.com/en-us/sharepoint/dev/sp-add-ins/get-to-know-the-sharepoint-rest-service?tabs=csom