@using BTCPayServer.Plugins.LiquidPlus.Controllers
@{
    var isActive = ViewContext.RouteData.Values.TryGetValue("Controller", out var controller) &&  controller is not null && 
                   nameof(CustomLiquidAssetsController).StartsWith(controller?.ToString(), StringComparison.InvariantCultureIgnoreCase);
}

<a class="nav-link @(isActive ? "active" : string.Empty)" asp-action="Assets" asp-controller="CustomLiquidAssets">Liquid Assets</a>