Step::1 Login to your WHMCS.
Step::2 Hover over the Setup section and click on General settings.
Step::3 Now jump on to the Ordering tab and scroll down to bottom of the page and look there for “Signup Anniversary Prorata” option.
Step::4 Tick on it to enable and Save Changes.
That's It !
Check out the video tutorial to get in-detailed procedure.Read the blog as well.