How do I add money to my iTunes account on my MacBook Pro running OSX10.13.6

I am using my older MacBook Pro running OSX 10.13.6. I have the iTunes app downloaded and have my self created iTunes library right on the MAC. I would like to add money from an Apple gift card to my iTunes account so I can download more music from the iTunes store. I can't seem to be able to find a

"Add money to my account" . When I did find it, and selected the amount of $25. the screen went grey and doesn't prompt me to give the serial number. It just shows the balance and prompts me to set up Auto Reload. But I don't want and AUTO reload. I want to add money when I deem I need more to cover the expenses of downloading music or other apps.

What am I doing wrong? Please advise.