Hi Bhaskar,
Which version of LiveCycle are you using?
Do you want to apply digital signature on XFA form created using LiveCycle Designer or Acroform created using Acrobat?
If you are trying to apply digital signature on XFA based form
You can create a text field with "Click Here to Sign" and overlay digital signature field on it and use Appearance Options of Digital Signature service APIs to set opacity of logo PDF to 100% to hide the text "Click Here to Sign" when user applies the signature
--Santosh
http://about.me/nskumar