You can avoid adding the
<%@ Register Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" TagPrefix="telerik" %>
directive at the top of every ASP.NET page by registering the Telerik controls globally in your project's Web.config file. This approach allows you to use the controls on any page without the need for the register directive:
To do that, open your web.config file and locate the system.web section:
<system.web>
<pages>
<controls>
<add tagPrefix="telerik" namespace="Telerik.Web.UI" assembly="Telerik.Web.UI" />
</controls>
</pages>
</system.web>
After this change, you can use <telerik:RadTextBox> and other Telerik controls directly in your markup without the need for the register directive on each page.