textarea {resize:none}

rptSearchList.DataSource = loPagedDataSource;
rptSearchList.DataBind();

//Initialize link button for ajax
foreach (RepeaterItem loItem in rptSearchList.Items){
   if (loItem.ItemType == ListItemType.Item || loItem.ItemType == ListItemType.AlternatingItem){
      LinkButton lbFavourites = (LinkButton)loItem.FindControl("lbFavourites");
      
       ScriptManager ScriptManager1 = (ScriptManager)FindControlRecursive(this.Page, "ScriptManager1");
       ScriptManager1.RegisterAsyncPostBackControl(lbFavourites);
   }
}

The above code first finds the LinkButton based on its ID within the repeater control. In my example I have added one further step as my Repeater was part of a UserControl sitting within a MasterPage. The MasterPage held the ScriptManager so I used a method for finding the ScriptManager from the user control. The below code is a useful method I used to find the ScriptManager.

public static Control FindControlRecursive(Control controlToStartFrom, string ctrlIdToFind){
   Control found = controlToStartFrom.FindControl(ctrlIdToFind);

   if (found != null)
      return found;

   foreach (Control innerCtrl in controlToStartFrom.Controls){
      found = FindControlRecursive(innerCtrl, ctrlIdToFind);

      if (found != null)
         return found;
   }
   return null;
}