How to set Focus to the Controls of webForm

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C#3.5

Code:

else if (answer is HandwrittenAnswer)
{
  this.Sourcecode.AppendLine("\t\t\t\t<textarea id='" + id + "' name='" + id + "' value='' style='position: absolute; "
                                                         + "top: " + top + "px; left: " + left + "px; height: " + height + "px; width: " + width + "px; "
                                                         + "visibility: hidden; z-index: 2; resize: none;'></textarea>");
  this.Sourcecode.AppendLine("\t\t\t\tdocument.getElementById (" + id + ").focus();");
  
}

with above code when i click on my web form it creates a Control. When i click firsttly it creates a control(a box) where i can write handwritten, but to write something in this control i need to click again because there is no focus ,first line of my code create a control now i want to set a focus so that when i click in it i can write something directly how to do it, for that i tried second line
this.Sourcecode.AppendLine("\t\t\t\tdocument.getElementById (" + id + ").focus();");
but it seems not working instead whole line is written on the Top of my webform..
??? Anybody got a solution???