Subscribe to receive in-app messages. The subscriber callback will be called whenever a new in-app message is
triggered. This method should be called before calling [[openSession]].
Parameters
callback: SubscribeToInAppMessageCallbackType
The callback function to handle the in-app message. This function will be
called with an [[InAppMessage]] or a [[ControlMessage]] object.
Returns string | None
The identifier of the subscription created. Returns undefined if the SDK has not been initialized.
Subscribe to receive in-app messages. The subscriber callback will be called whenever a new in-app message is triggered. This method should be called before calling [[
openSession]].