FlexPay's endpoints are located at:
https://api.flexpay.io/v1/gateways/charge
https://api.flexpay.io/v1/gateways/authorize
To initiate a request to any of FlexPay.io’s endpoints, the 'Authorization' field of your HTTPS header request must include the word "Basic" and an API key provided by the FlexPay Support Team or by logging into your FlexPay UI account and generating an API key. API keys can be used in either Sandbox or Live mode and a different set of keys is required for each business channel.
An example Header Request:
curl "https://api.flexpay.io/v1/transactions” -H "Authorization: Basic kIqCBL40PaoL2Tg28hUxVgtqKgYDTSLnl5J3iwptJHA="