Problem with control InputFormTextBox

Hi!
I use Visual Studio 2013. I found default InputFormTextBox in Sharepoint 2013. But control (*.Text) is empty when I pressed button. We want to edit html-formatted text.
Control on ASPX page:

<SharePoint:InputFormTextBox ID="rtbShort" runat="server" TextMode="MultiLine" RichTextMode="FullHtml" Columns="100" Rows="10" RichText="true"/>

Code behind for button:

string str = strShort.Text;