docs: describe mandatory env-var to call Platform tools for each integration (#3803)

docs/en/enterprise/integrations/slack.mdx

@@ -17,6 +17,38 @@ Before using the Slack integration, ensure you have:
 - A Slack workspace with appropriate permissions
 - Connected your Slack workspace through the [Integrations page](https://app.crewai.com/integrations)
 
+## Setting Up Slack Integration
+
+### 1. Connect Your Slack Workspace
+
+1. Navigate to [CrewAI AMP Integrations](https://app.crewai.com/crewai_plus/connectors)
+2. Find **Slack** in the Authentication Integrations section
+3. Click **Connect** and complete the OAuth flow
+4. Grant the necessary permissions for team communication
+5. Copy your Enterprise Token from [Integration Settings](https://app.crewai.com/crewai_plus/settings/integrations)
+
+### 2. Install Required Package
+
+```bash
+uv add crewai-tools
+```
+
+### 3. Environment Variable Setup
+
+<Note>
+  To use integrations with `Agent(apps=[])`, you must set the `CREWAI_PLATFORM_INTEGRATION_TOKEN` environment variable with your Enterprise Token.
+</Note>
+
+```bash
+export CREWAI_PLATFORM_INTEGRATION_TOKEN="your_enterprise_token"
+```
+
+Or add it to your `.env` file:
+
+```
+CREWAI_PLATFORM_INTEGRATION_TOKEN=your_enterprise_token
+```
+
 ## Available Tools
 
 ### **User Management**