To Create a Drop-down Button

Do one of the following:

Select File > Settings > User > Customize Ribbon .

or
Right-click in the blank space in the ribbon and select Customize Ribbon from the pop-up menu.

The Customize Ribbon dialog opens.
In the list below Customize the Ribbon, navigate to the group in which you want to create a custom drop-down button.
Click New Drop-down Button.

A new drop-down button is added in the selected group.
(Optional) In the Properties section, set the following:
1. To change the label of the drop-down button, enter the new label in the Label field.
2. To set the visibility, select the visibility expression from the Visibility drop-down list.
  Note: The Visibility property holds the name of a named expression that will be evaluated each time the active model is changed to determine whether the item should be displayed. The named expression should return a boolean (True or False) result in order to work properly as a Visibility property.
3. To set the enable/disable criteria, select the named expression from the Enable drop-down list.
4. Set other fields, as desired.
Click Apply.