West Germany's radical Green Party appealed to Soviet leader Yuri Andropov to allow Afghan self-determination. In a telegram published here, the party executive urged Mr. Andropov to show the seriousness of his peace proposals by ''making a genuine gesture of peace in Afghanistan.'' The Greens quoted reports of growing Soviet brutality against civilians. The party is best known for its strong stand against nuclear weapons. During a session in West Germany's Bundestag with 500 invited young people, a group of youths stormed the podium, threw paint, and raised antiwar banners before policemen dragged them away. A Greens spokesman said he did not immediately know whether they were people the party had invited.