GTK# : how to load widget at runtime

I placed a HBox (m_Hbox with 3 columns)  container on the MainWindow.

in the first column of the HBox i added one button (m_AddContenButton). in the third column I added another button (m_RemoveContentButton).

Started the application and it run fine.

My problem:

I created two custom widgets that I want to load into the second column of the  HBox (m_HBox) at runtime using m_AddContentButton_Click and remove it with m_RemoveContentButton_Click.

How can I do this ?

Thanks in advance for you Help