Submiting web form with desktop app

Question

Hello, I need help. I have the following code:

            InternetExplorer IE = new InternetExplorer();

            string URL;

            object o = null;

            object misValue = System.Reflection.Missing.Value;

            
            HTMLDocument HTMLDoc = new HTMLDocumentClass();

            URL = "http://www.google.com";

		//Navigates to site

            IE.Navigate(URL,ref o, ref o, ref o,ref o);

		//Makes IE visible

            IE.Visible = true;
            

	    do		

			//Waits until IE doesn't load page
            {
                Application.DoEvents();

            } while (IE.ReadyState != SHDocVw.tagREADYSTATE.READYSTATE_COMPLETE);

	    HTMLDoc = (HTMLDocument)IE.Document;

		//Locates the element on web document

            HTMLInputElement field = (mshtml.HTMLInputElement)HTMLDoc.all.item("q", 1);

            try

            {
                field.value = "1234"; //Sets value of field

            }
            catch (Exception e)
            {
                MessageBox.Show(e.Message);
            }

This code works perfectly for Google site.... However i don't need to fill Google forms. I need to fill 2 fields on this address:

https://www.zaba.hr/ebank/gradjani/Prijava 

As you can see this is https site. I don't know if there is some connection with this code and https but when change my code with:

//Wanted URL 

URL = "https://www.zaba.hr/ebank/gradjani/Prijava";

 AND

 HTMLInputElement field = (mshtml.HTMLInputElement)HTMLDoc.all.item("otp", 1); //Second field on form
            try
            {
                field.value = "1234";
            }

I get the following exception:

OBJECT REFERENCE NOT SET TO AN INSTANCE OF OBJECT

I don't know what am I going wrong :(

Please help me