Creating a Custom field in PayPal Payflow Link Credit Card Form

I need to add one custom field for Paypal Payflow link Credit Card form. I have searched and found the solution for adding custom field for Express Checkout. I have tried but I could not get the solution.

You could use one of the variables that are available with Payflow, such as comment1, comment2, user1, user2, user3, and etc. You should find these variables referenced in the developers guide.