@Hounder The Mangadex Bitcoin Wallet can be found on the https://mangadex.org/support page. You can either cut and paste the entirety of the address, or you can use a phones camera and phone based wallet app to register the QR-esq wallet code.
When you are CLAIMING your donation, thats a bit more complicated. You have to enter
YOUR wallets address.
The actual location of said address varies based on what youre using (an offline app, or an exchange) but basically there will be someplace that has a Wallet Id, which will look like a 30+ character string of completly random upper and lowercase letters and numbers. You will have to type that MANUALLY AND EXACTLY, making sure a capital letter is capitalized and a lowercase one, into the Support/History claim transaction box. You only should have to do this once, unless you use multiple wallets.