As far as I know, you can’t add them to the online version, if they aren’t there.
It is my understanding that the free basic version only has a limited number of styles.
from: https://clarivate.libguides.com/endnote_training/endnote_online
There are two versions of EndNote basic.
- The free version of EndNote basic has 21 styles and a limited number of filters and connection files. This version is available to anybody, with no other purchase required.
- The version of EndNote basic available as part of the Web of Science has thousands of styles and hundreds of filters and connection files. This version is accessible by those whose institution has a subscription to the Web of Science.