Hi Sander, if I understand your need correctly, you might try my Selection Helper widget:
You could develop a page similar to this.
The left hand side has a listview with datasource from database for a table called "conversation". To the right of it is a dataview that has a datasource "listen to widget" and is set to listen to the list view. Once an item is selected in the list view, then the dataview will populate with that items details.
For the new button, you can use a microflow button that creates an object of conversation and passes it to a popup page. The pop up page would have a dataview with details that would be filled out. The send button would be another microflow button, that would commit the conversation object and handle whatever other logic that needs to be done. Once the object is comitted, it will populate in the list view.
This is the simple domain model I used to make this page.
Here is the link to the mendix documentation. I would look into List views and dataviews.
as well as a link to the mendix learning paths to help you get started
You can try playing around with a template grid rather than a list view. Just set the template grid to have one column and a lot of rows. There is a setting called "select first", which will select the first object in the list. If you set up the list order to have the newest objects at the top, then the newly created conversation should be selected.