How to access to form variables created by reflection c#

Hi guys I´m trying to set a form variable that was created like this

var type = Type.GetType("namespace." + "formName");
Form form = Activator.CreateInstance(type) as Form;

The varible was created in the form designer

public int myVariable = 0;

What a I roughly want it to do is something like form.variable = 7 I tried this:

form.GetType().GetProperty(propertyName).SetValue(form, 7, null);

But it doesn't work :(

EDIT Sorry guys if didn't explain myself, I'll try to be more specific

        private void ShowForm(string frmname, string propertyName, int propertyValue)
    {
        var type = Type.GetType("RetailSystem.Compras." + frmname);
        Form form = Activator.CreateInstance(type) as Form;

        if (form != null)
        {
            // Set variables
            form.Show();
        }            
    }

This function will recieve a string which is the name of the form