The biggest question is if it's worth it to use a useless if statement to make the x and y variables local away from the main statement.

    //Unconventional part, make it so that x and y are local variables
if(true){
        int x,y;
        //set x and y to be the width + height of the terminal
        getmaxyx(stdscr,y,x);
        for(int i = 0; i < x; i++){
                //top border
                mvaddch(0,i,'#');
                //bottom border
                mvaddch(y - 1,i,'#');
        }
        for(int i = 0; i < y; i++){
                //left border
                mvaddch(i,0,'#');
                //right border
                mvaddch(i,x - 1,'#');
        }
}