Web Forms uses server controls.  Replace the link with a standard HyperLink server control which provides access to the NaviagtionURL property.

https://docs.microsoft.com/en-us/dotnet/api/system.web.ui.webcontrols.hyperlink?view=netframework-4.8

If you must use an HTML element then use a code block.

<a href='<%=YourPageProperty%>' id="Test">Test1</a>

    public partial class _Default : Page
    {
        public string YourPageProperty { get; set; }

        protected void Page_Load(object sender, EventArgs e)
        {
            YourPageProperty = "/About.aspx";
        }

    }