I’m not sure this is really possible through the Stripe integration. WHMCS is calculating the tax on its side, while Stripe is mainly receiving the final transaction amount. Passing extra metadata wouldn’t necessarily make Stripe treat that portion as tax for its own tax reporting or liability calculations. It would probably require deeper changes to how WHMCS and Stripe handle tax data, rather than just adding a few metadata fields.
Source: https://fnffree.io/