Customization Guide (.pdf) - Documentation & Online Help - Autodesk

A new drop-down (named New Drop-down) is created in the last position
of the row.
6 Right-click the new drop-down. Click Rename. Enter a new name.
7 In the Command List pane, drag the commands that you want to add to
the new drop-down in the Customizations In pane.
8 Adjust the properties of the drop-down as desired to control the
appearance of the drop-down and the commands under the drop-down.
on page 164
9 If the drop-down will use the Split behavior, add a command to the
Primary Command node under the drop-down to set the command for
the main button.
10 Click Apply.
170 | Chapter 4 User Interface Customization