Position a box in LaTeX-Beamer at the top of the slide

Question

I finally managed to create a box. I want this box to be centered and at the top of my slide, however. And I don't know how to achieve this: I have created a minipage, but I cannot position it at the top of my slide. I am using a customized template that does not allow for the block-environment.

\documentclass{beamer}
\usetheme{Hannover}

\begin{document}

    \begin{frame}{My Title}
        \setbeamercolor{postit}{fg=black,bg=green!30}
        \begin{itemize}
            \item Item1
        \end{itemize}
        \centering
        \begin{minipage}[t]{\textwidth}
            \begin{beamercolorbox}[sep=1em,wd=11cm]{postit}
                Put some text here.
            \end{beamercolorbox}
        \end{minipage}
        \begin{itemize}
            \item Item2
        \end{itemize}
    \end{frame}

\end{document}

Answer 1

Like this?

\documentclass{beamer}
\usetheme{Hannover}

\begin{document}

    \begin{frame}{My Title}
        \setbeamercolor{postit}{fg=black,bg=green!30}
        \vspace{-3cm}
        \begin{beamercolorbox}[sep=1em,wd=11cm]{postit}
            \centering
            Put some text here.
        \end{beamercolorbox}
        \vspace{3cm}
        \begin{itemize}
            \item Item1
        \end{itemize}
        \begin{itemize}
            \item Item2
        \end{itemize}
    \end{frame}
\end{document}