How can I make a Textbox a mandatory field if a checkbox is checked on a button click event in the client side?

<asp:TextBox id='TextBox1' style='Z-INDEX: 101; LEFT: 32px; POSITION: absolute; TOP: 104px' runat='server'></asp:TextBox>
<asp:CheckBox id='CheckBox1' style='Z-INDEX: 102; LEFT: 24px; POSITION: absolute; TOP: 80px' runat='server'></asp:CheckBox>
<asp:Button id='Button1' style='Z-INDEX: 103; LEFT: 32px; POSITION: absolute; TOP: 144px' runat='server' Text='Button'></asp:Button>

<script>
function func1()
{
	 if(document.getElementById ('CheckBox1').checked == true) 
	{ 
 		if(document.getElementById('TextBox1').value == '') 
		{ 
			alert('Enter something in textbox');
		 } 
	}
   
 }
</script>

VB.NET

Button1.Attributes.Add ('onclick' , 'return func1()');

C#

Button1.Attributes.Add ('onclick' , 'return func1()');