It's not possible to control the tab key like that. Once you're in the subform you can't make it go back to the 'main' form. Teach your users to use Ctrl+arrow down, that'll do the trick. Try it... open a sales order, put your cursor in the lines and press Ctrl+arrow down. This should put you right on the first button below the lines subform.
Comments
What are you trying to do ?
It's more user-friendly.
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