How to force the form focus?

Try this:

this.BringToFront();
this.Activate();

You need to show the form first - use the Show() method:

var form = new loginForm();
form.Show();

Edit: (updated question)

For an existing form calling Activate() might be more appropriate, this also brings the form to the front:

private void button1_Click(object sender, EventArgs e) 
{
   var form = new loginForm();
    if (Application.OpenForms[form.Name] == null) 
    {
           form.Show();
    } 
    else 
    {
        Application.OpenForms[form.Name].Activate();
    }
}

If the form is minimized you need to subscribe to the Activated event to change your window state to FormWindowState.Normal:

private void loginForm_Activated(object sender, EventArgs e)
{
    this.WindowState = FormWindowState.Normal;
}

it should be

private void button1_Click(object sender, EventArgs e) {
   var form = new loginForm();
    if (Application.OpenForms[form.Name] == null) {
           form.Show();
    } else {
         Application.OpenForms[form.Name].Focus();
    }
}