What is the encoding of that string?

I try to find the encoding of a string. I thought at first it was Base64, but actually not, it seems ...

Let me explain quickly. For a exercise, I have to decode the following string: ZXJpcjRxbnJ1YnZxbmU=

I have to use a form on a website (I can't distribute the URL) that allows me to generate an encoded string from what I entered.

For example, if I type the letter A, I have this: Tg== For B: Tw== And fortunately, the value remains the same (so no grain of salt).

Can you help me? I'm on it for a while and I still have not solved my problem..

Thanks. o/