Feature Request: external_id

[makinghappen]

Checklist before submitting a feature request

• [X] I've updated to the latest released version of the SDK
• [X] I've searched for existing Github issues
• [X] I've read the Code of Conduct

Goals

Library does not support passing external_id used for advanced matching Can it be implemented?

ios library already has it https://github.com/facebook/facebook-ios-sdk/issues/1496

Expected results

support external_id

Code samples & details

// INSERT YOUR CODE HERE

[Solracjg]

#1208