creating a key for another form....(please help!!)

[andy101]

I have a continuous from which allows the user to enter details of an assessment, some times the user needs to enter more details about a particluar area of the assessment on another form. On the main assessment form i have a field called plan ID (this number is to be the primary key on the other details form). I want to be able to press a button which will automatically populate the plan ID field with a number (this number has to be unique, the default value is 0), I then want to be able to press another button which will open the other form with the plan ID already in the plan ID field. The problem is that more details are not always needed for every area of the assessment. Please Help!!