basically i want to add a card for a customer using API and then this card will be charged programmatically
You can use the Cards API to save a credit or debit card for a customer. Your application can then charge the card when the customer makes purchases in the future. 
ok you means once card added then our application can charge this card programmatically without going to a form
and can you explain what is the source_id on createCard API endpoint ?
because we have Billing application and we want to charge cards automatically at due dates without involving customer
Yes, once the card is added to the account you can charge the customer programmatically. The source_id is the tokenized card from the Web Payments SDK that’s used to save the card on file.