You can do that with Concatenation

Set the field name = to the LEFT function to get the three characters of item category, concatenate it with the format of the date, concatenate with the IDItem

LEFT is covered in expert 14 which you have.

There was a very similar example to this presented in the Customer Codes video

That does not work.  I would like to do this in the form.  Using the form fields and storing them in the tbl.  I can use a two column Category Table, [Prefix] and [Category] and display category in form and use prefix as part of concatenation.  Any ideas? Tnx.

Is this what you are trying to do?
ItemKeyID = Left(ItemCategory, 3) & Format(PurchaseDate, "yymmdd") & IDItem

Why would you store it in the Table when it can be calculated?

Kevin Robertson yes that is what I am trying to do.  does not work.

After taking another look at your screenshot, ItemCategory is in a combo box you you will probably need to access the column property (If ItemKeyID is being generated directly on the form).
ComboName.Column(1)