Nextiva / Asterisk -- Integration (Stage 1 - Completed)

Folks,

I have been working with Asterisk for a few years now, but am I total newbie for sure.
I am running Asterisk 1.8.24.0

Using MySQL realtime for queues, dial plan, SIP users, etc.
CDR data is also going to MySQL for reporting.

A customer I do work for has a multiple lines/extensions from Nextiva.

Over the last few days, I have successfully setup one of their Nextiva extensions on my Asterisk PBX.
Works with no issues. Inbound or outbound calls are working great.
This was stage 1.

Stage 2 is where I have a few questions/concerns —

When a call is active and using this Nextiva/SIP trunk — How do I / How can I — send feature codes/commands to the nextiva system?

Example ---- ----- ----- -----
Remember my Asterisk Server is a SIP Client for an existing Nextiva customer.

When I put them on hold or transfer the call I need to send the command back to the Nextiva system and have the action done at that point and NOT at my PBX level.

I hear Nextiva has a API but it sounds like its a guarded secret.
I was thinking about if I could push a STAR code or command over the Nextiva trunk or something.

Has anyone done this?
What are my options?

Thanks
Matt

From what I can tell —

Broadsoft API is what I need to use.
I looked at their developer WIKI and most links/pages are “Not found”.

Has anyone worked with BroadSoft API ???