Change HyperLink NavigteURL on click using a code behind method

Hi,
I  have a simple WebForm where there is a text box and a HyperLink next to it. Initially the NavigateUrl is to be empty.
Once the user types something in the textbox and then click the hyperlink, I need to call a server side method which will take the input of the textbox, manipulate it as necessary then set that as the NavigateUrl of the link. The target for the hyperlink is "_blank" so it will open another window or tab so the original page where the hyperlink is, I then need to clear the NavigateUrl for the next usage.
Any help with this would be appreciated.
Cheers :)