> For the complete documentation index, see [llms.txt](https://help.clickatell.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.clickatell.com/developers-api-reference/external-link-callback.md).

# External Link Callback

## Direct an end-user to an external web page

> The External Link Handler API allows you to direct an end-user to an external web page from within a Flow conversation and then return to the Flow conversation with optional additional data gathered from the external web page.\<br />\<br/>The end-user can then continue with the conversational workflow according to the steps defined in Flow.

```json
{"openapi":"3.0.1","info":{"title":"Flow","version":"1.1.0"},"servers":[{"url":"https://flow-callback.clickatell.com/","description":"Production server"},{"url":"https://control-callback-uat.payd.co/","description":"Sandbox/UAT server"}],"paths":{"/callback":{"post":{"tags":["External Link Callback"],"summary":"Direct an end-user to an external web page","description":"The External Link Handler API allows you to direct an end-user to an external web page from within a Flow conversation and then return to the Flow conversation with optional additional data gathered from the external web page.<br /><br/>The end-user can then continue with the conversational workflow according to the steps defined in Flow.","parameters":[{"name":"reference","in":"header","description":"A reference to be passed as a header. This value is also required in the body section, please refer accordingly for the description.","required":true,"schema":{"type":"string"}}],"requestBody":{"description":"Payload describing the external link","content":{"application/json":{"schema":{"$ref":"#/components/schemas/Externallink"}}},"required":true},"responses":{"200":{"description":"Successfully validated the response and returned data appended to matched session.","content":{}},"400":{"description":"A bad/invalid reference was returned and the session could not be matched","content":{}}}}}},"components":{"schemas":{"Externallink":{"required":["result"],"type":"object","properties":{"result":{"type":"string","description":"Used to evaluate success or failure in Flow and can be any value you wish to return for evaluation"},"reference":{"type":"string","description":"A unique hash generated by Flow based on SessionID and SourceID that allows us to identify the user's session. This value is automatically appended as a query parameter on external links and should be intercepted and returned with the callback for validation (in header and body): e.g. `https://www.example.com?reference=f7_4hn5=8f&%#nK=`"},"data":{"type":"object","additionalProperties":{"type":"string"},"description":"An optional hashmap with any key and value that can be referenced in the flow later on."}}}}}}
```


---

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