It’s important to distinguish the difference between WordPress.com and WordPress.org, as they are not the same thing. The first one offers hosting services and restricts users from installing plugins which are not included in the list of plugins from WordPress.
WordPress.org, however, is an open source downloadable software which allows unlimited customization with external plugins and features on the user’s private server (FTP). If you’re using a blog on a server hosted by WordPress, you won’t be able to install the 123FormBuilder plugin at all .
In case you do use the WordPress software from WordPress.org to administrate and publish articles from a private server, log in to your WordPress account and look for the Plugins tab located in the collapse menu on the left, under Appearance. Once the Plugins tab is found, follow our documentation on how to install the 123FormBuilder plugin on WordPress.
You can also go to the WordPress plugin directory and install the 123FormBuilder plugin from there.