Have a blackboard vector for tracking player location and update it the tick event of your Behavior Tree service.
The provided MoveTo node is not good for most purposes, because it doesn’t return true until the character reaches their destination. Instead, you want to create a task that calls AIMoveTo on the character’s AIController, but then returns immediately. That will cause them to move toward the destination, but will allow your behavior tree to continue. Next pass through, it will call MoveTo with the updated location. The result will be smooth tracking toward the player.
If you’re not scared of a little C++, I wrote a Behavior Tree task called “Move Toward” that works exactly like the built-in MoveTo node, except that it returns immediately. You can find it in this thread: https://forums.unrealengine.com/showthread.php?50057-BTTask_MoveToward-Asynchronous-quot-Move-To-quot-Behavior-Tree-task&highlight=MoveTo