In most cases, you do not need to obtain the instance ID. If you are using a Show Form button within a table, PerfectForms™ stores the child instance ID as a property of the button.
The instance ID of the current form instance can be found within the Formula Editor. The Formula Editor is accessed by behavior commands such as the Set Field command.

When you click the Compose button, the Compose Formula dialog is displayed. Select the Special option and you can then select the instance ID or the parent instance ID.

Updated: 06/12/2011
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http://www.perfectforms.com/Documentation/manual/html/?getting_at_the_form_instance_i.htm
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