Hi. MFA only available for Plesk entities, i.e. Plesk admin / reseller / customer / smb_user (anyone who can log into Plesk UI).
Since the Webmail user does not fit in that paradigm, the only option is to configure MFA manually, e.g. RoundCube provides a plugin for that - How to setup 2FA for Roundcube | RoundcubePlus - Webmail Skins and Plugins
If we speak about Plesk for Linux, "Plesk Webmails" - it could be Plesk Premium Email, Roundcube, Horde, SOGo - all these webmails provide (or do not provide) the ability to protect accounts with 2FA independent of Plesk itself.
As an example,
Roundcube does not provide the ability to configure 2FA by default (but some plugins add this ability)
SOGo Webmail provides the ability to configure 2FA. (inside the webmail interface: Preferences / General / Enable two-factor authentication using a TOTP application)