how can open a new tab using link button not hyper link in c#

Why do you need C# code for that, when it can be easily handled using the client-side language of JavaScript? Always try using the easy approach not the hard way of achieving something, C# code won't be as efficient as a JavaScript code that would open a new tab for your website with specified URL in it.

Try the following code in your JavaScript language (write this inside script element of your HTML markup)

window.open("<url>", "<windowname>");

For more on this, please read this MDN document[^].