For an xgboost model, you'll want to use WIT's custom prediction function ability. See
https://pair-code.github.io/what-if-tool/learn/tutorials/custom-prediction/ for more information. You should be able to define your own python function that does the prediction on a list of input examples and returns a list of outputs for those examples. That function can make use of your trained xgboost model.
Another option is to deploy your xgboost model to Google Cloud AI Platform Prediction and use WIT's ability to query models served through AI Platform Prediction. But the custom prediction function will be a much simpler approach for your use case most likely.