To create FTP user through SSH you would need to follow below mentioned steps:

1. Login as root through SSH.

2. Next add the user account you want using the ‘useradd’ command

useradd <username>

3. Now create a special group for that user.

groupadd <groupname>

4. Now to add the user to the group

gpasswd -a <username> <groupname>

These commands are non-standard but available on most popular distributions.
If not, then you can try editing /etc/group using your favorite text editor.

5. Change the group ownership of the special directory to that group.

chgrp -R groupname /path/to/your/web/directory

6. Enable write permissions

chmod -R g+rw /path/to/your/web/directory