I got this question on my post about using the PHP Client library for API v2.
I think there is some confusion among my readers about using the REST API when it comes to using and managing contexts.
But before we go any further, I will mention that I don't recommend using slot filling. Read why here.
However, if you still wish to use slot filling via the REST API, you need to understand the role that the sessionID plays in REST API calls. I have discussed this in my post on tips for Dialogflow REST API users.
Here is a video which summarizes my views on this topic.
In the Dialogflow REST API v2 course, I provide a fairly in-depth, programming-language-agnostic guide to using REST API v2 in Dialogflow. For example, you will find it much easier to implement the material covered in this post by going through my course.
Check out my guide to Dialogflow slot filling.