All Limesurvey Versions are somehow "web versions", it is a web application that needs to run on a web server. So "web version" doesn't say much. I assume that you are referring to the SaaS version hosted by the Limesurvey GmbH (vs selfhosting).
As far as I know, you can not install plugins and addons on the SaaS version yourself. You will need to get in touch with the support (go to the website and open a support ticket, most respondents here in the forum are users that help users, we have no insights into the internal process of the Limesurvey GmbH).
I answer at the LimeSurvey forum in my spare time, I'm not a LimeSurvey GmbH employee.
No support via private message.
