Any developers can integrate their applications with Alpaca API via Alpaca OAuth. Once an application has been approved by Alpaca for third-party live trading, you will be able to offer commission-free trading inside your application.

The OAuth integration is the easiest way for you to test if your users are interested in the stock trading capability as a part of your application. Read more here.

OAuth Integration Guide - Documentation | Alpaca
Alpaca supports OAuth for anyone who builds apps and services using trading API. Alpaca API lets you build and trade with real-time market data for free.
What is OAuth Support? An Easy Way to Offer Commission-Free Trading Directly on Your App!
Add Alpaca’s commission-free trading capability inside of your app with the minimum effort through a commonly used integration method of OAuth.