How to Create Custom APIs for Power Automate

Overview

Power Automate provides many built-in connectors, but sometimes businesses need custom APIs to extend its functionality. By creating a Custom API, you can:

Connect Power Automate to internal applications
Expose business logic via REST endpoints
Enable authentication and security control
Enhance automation workflows with external data

This guide covers how to create a Custom API for Power Automate using Azure API Management and Azure Functions, and then expose it as a custom connector in Power Automate.

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1️⃣ Prerequisites

Before creating a custom API, ensure you have:

✔️ Azure Subscription (for API hosting)
✔️ Power Automate License (for custom connectors)
✔️ Postman (for API testing)
✔️ Basic knowledge of REST APIs and JSON

Example: You want to create a Custom API that retrieves employee details from an internal HR system and use it in Power Automate.

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2️⃣ Step-by-Step: Creating a Custom API for Power Automate

Step 1: Create an API Using Azure Functions

Azure Functions provide a serverless way to build lightweight APIs.

1️⃣ Go to Azure Portal → Search for Azure Functions
2️⃣ Click Create and configure:

• Resource Group: CustomAPI-RG
• Function Name: EmployeeAPI
• Runtime Stack: .NET, Node.js, or Python
• Hosting Plan: Consumption (serverless)
  3️⃣ Click Review + Create

Step 2: Develop the API Endpoint

Modify the function to return employee details in JSON format.

Example Function (C#/.NET):

using System.Net;
public static async Task<HttpResponseMessage> Run(HttpRequest req, ILogger log)
{
    string employeeId = req.Query["id"];
    var employeeData = new {
        Id = employeeId,
        Name = "John Doe",
        Position = "Software Engineer",
        Department = "IT"
    };
    return new HttpResponseMessage(HttpStatusCode.OK) {
        Content = new StringContent(Newtonsoft.Json.JsonConvert.SerializeObject(employeeData),
                                    System.Text.Encoding.UTF8, "application/json")
    };
}

4️⃣ Deploy the function and copy the Function URL

Example API Call:
GET https://employeeapi.azurewebsites.net/api/GetEmployee?id=12345

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Step 3: Secure the API with Azure API Management

To control access, Azure API Management (APIM) is used.

1️⃣ Go to Azure Portal → Search for API Management Services
2️⃣ Click Create and configure:

• Resource Group: CustomAPI-RG
• Name: HR-API
• Pricing Tier: Consumption
  3️⃣ Click Review + Create

4️⃣ Import the Function into APIM:

• Navigate to APIM → APIs
• Click + Add API → Choose Function App
• Select EmployeeAPI

5️⃣ Enable Subscription Keys (for authentication)

• Under API settings, enable Subscription Required

Example API Gateway Endpoint:
https://hr-api.azure-api.net/GetEmployee?id=12345

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Step 4: Test the API with Postman

1️⃣ Open Postman
2️⃣ Make a GET request to the API:

GET https://hr-api.azure-api.net/GetEmployee?id=12345

3️⃣ Add Headers:

Ocp-Apim-Subscription-Key: YOUR-API-KEY

4️⃣ Verify the JSON response

📌 Expected Output:

{
   "Id": "12345",
   "Name": "John Doe",
   "Position": "Software Engineer",
   "Department": "IT"
}

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Step 5: Create a Custom Connector in Power Automate

To integrate Power Automate with the API, create a custom connector.

1️⃣ Go to Power Automate → Data → Custom Connectors
2️⃣ Click + New custom connector → Import from OpenAPI
3️⃣ Provide:

• Connector Name: HR Employee API
• API Base URL: https://hr-api.azure-api.net/
• Authentication: API Key (use APIM key)

4️⃣ Define Actions in Power Automate:

• Click + New Action
• Name it Get Employee Details
• Set method to GET
• URL path: /GetEmployee?id={id}
• Define response schema using JSON sample

5️⃣ Save & Test the Connector

Example Power Automate Flow:

• Trigger: “When a new request is created in SharePoint”
• Action: “Get Employee Details” (using the custom API)
• Action: “Send email with employee data”

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3️⃣ Best Practices for Custom APIs in Power Automate

✔️ Use API Management (APIM) for security – Protect APIs from unauthorized access.
✔️ Enable CORS (Cross-Origin Resource Sharing) – Allow Power Automate to call the API.
✔️ Implement OAuth 2.0 authentication – Secure sensitive API endpoints.
✔️ Optimize API response times – Avoid large payloads and use pagination.
✔️ Log API requests and errors – Use Application Insights for monitoring.
✔️ Version control APIs – Maintain backward compatibility when updating APIs.

Example: If your API serves financial data, ensure role-based access control (RBAC) is implemented in APIM.