.net show form in MDIParent

Put the following code in to your MDI parent form

There are two methods to show form in MDIParent

Method 01

Dim form As New Form1()
form.MdiParent = Me
form.Show()

Method 02

'Display the Add form
 Form1.MdiParent = Me
 Form1.Show()

double click on add reservation (the menu button that you have created)

then 

add above code and change form name as your form