The entirety has a lot going on, but the important bits are like this:
'Stuff
Private WithEvents OriginConstrainCmd As Inventor.ButtonDefinition
Private WithEvents OriginConstrainCmdNX As Inventor.ButtonDefinition
Private WithEvents OriginConstrainCmdNY As Inventor.ButtonDefinition
Private WithEvents OriginConstrainCmdNZ As Inventor.ButtonDefinition
'More stuff
OriginConstrainCmd = objControlDefs.AddButtonDefinition("Origin Constrain", "OriginConstrainCmd", CommandTypesEnum.kQueryOnlyCmdType,,, "Constrains origins of selected parts to first selection.")
OriginConstrainCmdNX = objControlDefs.AddButtonDefinition("Exclude X Plane", "OriginConstrainCmdNX", CommandTypesEnum.kQueryOnlyCmdType,,, "")
OriginConstrainCmdNY = objControlDefs.AddButtonDefinition("Exclude Y Plane", "OriginConstrainCmdNY", CommandTypesEnum.kQueryOnlyCmdType,,, "")
OriginConstrainCmdNZ = objControlDefs.AddButtonDefinition("Exclude Z Plane", "OriginConstrainCmdNZ", CommandTypesEnum.kQueryOnlyCmdType,,, "")
'More Stuff
Private Sub addin_OnLinearContextMenu(SelectedEntities As ObjectsEnumerator, SelectionDevice As SelectionDeviceEnum, LinearMenu As CommandControls, AdditionalInfo As NameValueMap)
LinearMenu.AddButton(OriginConstrainCmd)
LinearMenu.AddButton(OriginConstrainCmdNX)
LinearMenu.AddButton(OriginConstrainCmdNY)
LinearMenu.AddButton(OriginConstrainCmdNZ)
End Sub
Commands themselves work fine, but on the Linear Menu object trying to use the other methods (AddButtonPopup, AddPopup, AddComboBox, AddSplitButton) all fail with a 'not implemented' error