Hi,
You will likely need to add a text AdaptationSet and Text representation to your manifest.
https://developer.bitmovin.com/encoding/reference/postencodingmanifestsdashperiodsadaptationsetsrepresentationschunkedtextbymanifestidandperiodidandadaptationsetid
The Text representation should reference your WebVtt stream
Please let me know in case you have any questions.