Rule Definition
STAThreadAttribute indicates that the COM threading model for the application is single-threaded apartment. This attribute must be present on the entry point of any application that uses Windows Forms; if it is omitted, the Windows components might not work correctly. If the attribute is not present, the application uses the multithreaded apartment model, which is not supported for Windows Forms.

Remediation
Add the STAThreadAttribute attribute to the entry point i.e. static void main method.

Violation Code Sample

using System; 
using  System.Windows.Forms;

namespace UsageLibrary
{
    public class MyForm: Form
    {
        public MyForm()
        {
            this.Text = "Hello World!";
        }
        
        //VIolation
        public static void Main()
        {
            MyForm aform = new MyForm();
            Application.Run(aform);
        }
    }
}

Fixed Code Sample

using System; 
using  System.Windows.Forms;

namespace UsageLibrary
{
    public class MyForm: Form
    {
        public MyForm()
        {
            this.Text = "Hello World!";
        }
        
        [STAThread] //Violation Fixed
        public static void Main()
        {
            MyForm aform = new MyForm();
            Application.Run(aform);
        }
    }
}

https://docs.microsoft.com/en-us/visualstudio/code-quality/ca2232?view=vs-2019