Can't add Bedrock players to the whitelist of a server (GeyserMC crossplay server)

Use the official solution from GeyserMC FAQ:

1. Turn off the whitelist using /whitelist off, then get the Geyser player to join.

2. After that, run /whitelist add ".<bedrock_username>".

3. Turn the whitelist back on using /whitelist on. When using Local Linking: If you’re linking a Bedrock account to an existing whitelisted Java account, you don’t need to whitelist the Bedrock account separately. You can link the accounts and immediately turn the whitelist back on.