Receiving error while trying to implement imessagebox and inject it to classes that displays messages boxes C#

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C#3.5

namespace AppModule
{
public interface IMessageBox
{
void ShowMessageBox(String msg);
}

public partial class Form1 : Form
{

private readonly IMessageBox _messageBox;
public Form1(IMessageBox messageBox)
{

this._messageBox = messageBox;
}

public void Doit()
{
this._messageBox.ShowMessageBox("Test");
}
}
}

Error Message : An unhandled exception of type 'System.NullReferenceException' occurred in AppModule.exe

Additional information: Object reference not set to an instance of an object.

What I have tried:

I do know that messagebox is null but I don't understand how to initialize it. Pls help