Question: Just what the title says. Suppose I know the feature that I want to be used for splitting for the root node in a tree model in XGBoost; is there a way for me to tell XGBoost to split on this feature at the root node?

Answer: 
It's hard to prove a negative, but I think the answer is no. Especially in xgboost compared to other tree-model packages, it's rather hard to even access the base trees, let alone modify their build.
Could you enforce this using [feature interactions](https://xgboost.readthedocs.io/en/stable/tutorials/feature_interaction_constraint.html)? Maybe create two sets of features with only that one feature in common? I’m not sure if it will guarantee that that shared feature is the one on which the first split is made.