When creating a custom form, open the Edit menu, and choose Tab Order to open the Tab Order editor.

Use this editor to specify the tab order of your custom form: the order in which all controls will be selected when a user presses the TAB button in the runtime version of the form. The Tab Order editor shows all visual controls present in the form of which the tab order can be set. This concerns all controls having the TabOrder property. The TabOrder property value specifies the tab order of the control within its container. The list is sorted by TabOrder. If desired, you can update the tab order number of a control. If you have selected a container control, e.g. a Panel, only the container's child controls will be displayed so that they can be updated.