You can implement a custom configure options for the member options

public class ConfigureCustomMemberCookieOptions : IConfigureNamedOptions<CookieAuthenticationOptions>
{
    public void Configure(string name, CookieAuthenticationOptions options)
    {
        if (name == IdentityConstants.ApplicationScheme || name == IdentityConstants.ExternalScheme)
        {
            Configure(options);
        }
    }

    public void Configure(CookieAuthenticationOptions options)
    {
        options.AccessDeniedPath = "/Clients/Login?redirecturl=";

    }
}

And then register it in the container, e.g. like this in startup.cs

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddUmbraco(_env, _config)
            .AddBackOffice()
            .AddWebsite()
            .AddComposers()
            .Build();

        services.ConfigureOptions<ConfigureCustomMemberCookieOptions>();

    }