Set default value in lookup field after submission

If I have a lookup field (dropdown) with countries, is there any way when I select a country and click submit then automatically set this country as default? . Can you please send me an example of this?

I tried (in On Load) to fetch records like

value = Country [Name_of_Country = "Germany"].ID;

Lookupfield.select(value);

With a country like Germany it works but I do not want this.