placeholder attributeYou're looking for the placeholder attribute. Use it like any other attribute inside your ASP.net control:
<asp:textbox id="txtWithHint" placeholder="hint" runat="server"/>
Don't bother about your IDE (i.e. Visual Studio) maybe not knowing the attribute. Attributes which are not registered with ASP.net are passed through and rendered as is. So the above code (basically) renders to:
<input type="text" placeholder="hint"/>
placeholder in resourcesA fine way of applying the hint to the control is using resources. This way you may have localized hints. Let's say you have an index.aspx file, your App_LocalResources/index.aspx.resx file contains
<data name="WithHint.placeholder">
<value>hint</value>
</data>
and your control looks like
<asp:textbox id="txtWithHint" meta:resourcekey="WithHint" runat="server"/>
the rendered result will look the same as the one in the chapter above.
Like any other attribute you can add the placeholder to the AttributeCollection:
txtWithHint.Attributes.Add("placeholder", "hint");