This needs to use the l10n setup. If you define a `data-l10n-id` attribute of something like `nickname_input_text` and then in `browser/locales/en-US/chrome/browser/loop/loop.properties` add `nickname_input_text.placeholder=...` that should do the expected.

I think we also want something shorter here, e.g. `Identify this call` or something like that.