Based on the discussion here, I think I am getting it now.
What I need for the GraphQL query is the hex address, whereas the Fuel Wallet displays only the Bech32 address (and there’s no way to view the address as hex, unfortunately).
But it looks like I can export the private key using the Fuel Wallet. Is there any CLI tool that I can use for deriving the public key?
Edit: I tried to generate the public key using this browser-based ED25519 tool, but the generated value didn’t seem to match my actual address (the query returned a balance of zero, whereas I have ~0.449 ETH on Testnet Beta 3).