When you have selected messages and messages_postback permissions at the time of App Review on Facebook Business Manager, you will then be asked to add the platform. The platform here is the platform where your bot is already deployed. If you have not deployed your bot anywhere else, you can select Website as the platform and add the demo link of the same (you can find the demo link on the Haptik platform).
Once done with this step, you must submit your app for review. You will have to add the steps that are required in order to navigate through your bot. For example, you can mention the process of working on your bot, like clicking on the lets chat icon on the bottom right side of the screen, then writing "hello" to start the conversation with the bot, and so on...
You should mention the general flow of conversation on the bot where the bot collects users' data.