I use the VS Code LanguageTool Linter extension and I had difficulties setting it up to use my premium account rather than the public account. I thought others might find it useful to know how I made it work.
Here are the only key configuration items for using the free public API:
Language Tool Linter: Service Type -> public
And here are the same configuration items to use the premium API:
Language Tool Linter: Service Type -> external
Language Tool Linter › External: Api Key -> my-secret-api-key-use-your-own
Language Tool Linter › External: Url -> https://api.languagetoolplus.com/
Language Tool Linter › External: Username -> my-private-email-address-use-your-own
I was inspired by another topic in this forum where they indicated we should remove the /v2 path segment from the URL that is used by other integrations such as obsidian and LibreOffice.
I hope this helps other readers of this forum.