There is a new blog by @gblumenthal and a companion wiki page that walks thru creating a new extension only using the API module in FreePBX 15.
blog: https://www.freepbx.org/freepbx-graphql-provisioning-tutorial/
wiki: https://wiki.freepbx.org/display/FDT/FreePBX+GraphQL+Provisioning+Tutorial
Thanks Greg!