C# Better structure in my code & any other critiques
Two assumptions (because it is simplet for me with them)

Code works as it should
This code should be prepared to go &quot;the enterprise road&quot; (e.g. we assume many people maintaining it for a long time in the future).

Note belows are opinions only, please do not treat is as a source of truth because your company might have a different guidelines. Since it is C# code I'm trying to stick to MSFT gudielines and, where not possible, with my own preference.
Overall comments

I'm really happy that you provided a working 'ready to run' example.
I think you should start thinking about unit testing your code if you haven't already. Simple unit tests as a start. For example 'does Upvote() increases the score`'.
I really like 'make your functions as short as it is convinient and logical' approach to writing code. Here, it would mean that ideally we would split the main function to a couple of small ones. I would be very happy if you refactored it to small logical functions and posted a new question so we can pick it up from there. I would suggest something similar to the below.

Main(){
    WriteWelcomeMessage();
    var userInput = ReadUserInput();
    var newPost = CreateNewPost(userInput);
    while(val){
        DisplayPost(newPost);
        userInput = ReadUserInput();
        ExecutePostAction(userInput, newPost);
    }
}

Or something similar, I think you get the idea. This can be further split so Main has only two or one function calls.
I consider this approach an implementation of separation of concerns and single responsibility principle.

This is still early in the project but if you would proceed with this as an enterprise application I would suggest reading on dependency injection. Here you can inject classes like reader, writer, postFactory or similar. This will make the testing much simpler in the future.
I highly recommend to include some style checker (e.g. stylecop) this will ensure that you can spot more guidelines issues.

Main function

Don't couple your implementation to Console class. I personally would write a class that would encapsulate writing/reading (or maybe two classes?) so you can easily replace it with for example reading from a WPF textbox or writing to a file.

I strongly believe in descriptive variable naming, variable always should be describing its content. Below variables are IMHO missnamed:

post - should be 'postContent'
val - shouldContinue (?)
inputData - userChoice


If you would rename the variables to be descriptive, you can drop type declarations. string or bool are not giving much context. See here for more info.


Post class

I like that you encapsulated upvoting and downvoting to functions.
Is there any reason why content of the post is not passed through the constructor? This might lead to null reference exception if the user of this class would forget to call AddPostMessage.
I would add private infront of the fields
You could inject a writer class to Post in order to decouple it from Console class.
You could also inject some DateTime provider in order to unit test this class easier.
I prefer using DateTime.UtcNow instead, this makes handling multiple timezones easier.