Send / add PSN credit to another PSN account?

I still have 20 euro psn credit on my old psn account, but now I use a new psn account and want to have the money on the new psn account. I still have access to the old psn account. Do you know how to send it over / transfer it or something like that, so that I can use the credit. Would be very grateful if someone could help me

It does not work. You can't transfer funds. You have to use it on the old account!

As far as I know, this doesn't work, but if you buy a game you have activated on your primary account, you can also use the game on the other acc, but this does not count for in-game purchases and mostly for add-ons