I need to convert the following link into absolute url form. Please help me to change this link.
css/bootstrap-datepicker3.css
You should just be able to type:
/css/bootstrap-datepicker3.css
If the "css" directory is within the root of your website, if it is not then you need to direct to that path, for instance if you have to go inside an "assets" folder to get to your "css" folder, the path would be:
/assets/css/bootstrap-datepicker3.css
Hopefully this answers your question.