Sure @kmoens,
I'll break it down to you below. These are basic examples, please refer to the documentation to learn more about all the options and parameters you can use. The Direct Line endpoints may also be different by region, but this is also documented.
#1 Generate a Direct Like Token:
The first thing you want to do is generating a Direct Line token for the conversation.
You can get it under Settings > Channels, in "Mobile App":

So just do a GET on the URL, and it will return the token as well as a conversationid.
E.g. in Postman for my test bot endpoint:
https://c53bf00279234d1cb5ae3265093d59.e1.environment.api.powerplatform.com/powervirtualagents/botsbyschema/cr507_testChatbot/directline/token?api-version=2022-03-01-preview

#2 Initiate Conversation
Then, still using Bearer Token authentication with the retrieved token in step #1, do a POST on:
https://directline.botframework.com/v3/directline/conversations
This will also return the streamUrl if you want to use WebSocket (Receive activities from the bot - Bot Service | Microsoft Learn)
E.g. in Postman:

#3 Send startConversation event
You can trigger the automatic start of the conversation by sending the startConversation event
Here, still using Bearer Token authentication with the retrieved token in step #1, do a POST on this URL.
Notice that now you also need to pass the conversationId in the URL:
https://directline.botframework.com/v3/directline/conversations/{{ConversationId}}/activities
In the body, send:
{
"type": "event",
"name": "startConversation"
}
E.g. in Postman:

#4 Send message
To send a message, still using Bearer Token authentication with the retrieved token in step #1, and still with the conversationId, do a POST on this URL:
https://directline.botframework.com/v3/directline/conversations/{{ConversationId}}/activities
In the body, send:
{
"type": "message",
"text": "When are you closed"
}
E.g. in Postman:

#5 Get responses
You can return all the chatbot activities (send and received messages), still using Bearer Token authentication with the retrieved token in step #1, and still with the conversationId. Do a GET on this URL:
https://directline.botframework.com/v3/directline/conversations/{{ConversationId}}/activities
E.g. in Postman:

Let me know if this helps
Henry