Set Folder Metadata Values

using Microsoft.SharePoint.Client;
using Microsoft.SharePoint.Client.DocumentManagement;

// Replace all instances of < > with your variables
using (var context = new ClientContext(<siteUrl>))
{
    context.Credentials = new SharePointOnlineCredentials(UserName, SecurePass);
    listToOperate = context.Web.GetListByUrl(<listRelUrl>);
    context.Load(listToOperate);
    context.ExecuteQuery();
    
    //Create the folder first
    Folder folder = listToOperate.RootFolder.EnsureFolder(folderName);
    context.ExecuteQueryRetry(5);

    //Setting the Metadata Defaults for the Folder
    MetadataDefaults metadataDefaults = new MetadataDefaults(context, listToOperate);
    metadataDefaults.SetFieldDefault(folder, "<ColumnName>", "-1;#" + <taxonomy values>);
    metadataDefaults.Update();
    listToOperate.Update();
    context.ExecuteQuery();
}