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Add a Category Combo Box to your Order Detail Subform so you can limit the product before selection.

You might check out Seach As You Type.

After re-reading your question, Cascading Combo Boxes might be more like what you are looking for.

In my old job we had tens of thousands of products, and the user simply typed the product ID for instant entry.  There was no way to navigate God knows how many levels of combo boxes to get to one of 10k+ products.  Many industries also require product IDs to be used.  That is not the same as UPC, which is a public-facing code to be used for checkout lines and such.  A "product ID" is often an internally used code, often made for convenient data entry and communication.  You want to say "Do we have any product# 1234P in stock?" Instead of "Do we have any Category A, sub-category B, sub-category C, ..., in stock?"

Appreciate the comments. I'm going through some search box navigation and will try the cascading combo boxes.
Right now our categories are kind of all over the place and not set for every product. Most of the information is in the item description which is quite descriptive. I was watching search form 2.0 and that helped as it would requery and filter the list which was very useful. I just need to combine it with combo boxes. I have set up an inventory list to open when working on an order but I don't know how to have the selected item moved/added to the product list in the order.

I got my Order forms close to the invoice tech help extended video but instead of having the combo box to add inventory items to the order it opens up the inventory list form. From the inventory list form there is a button that adds the selected inventory item to the order subform similar to the invoice. This code works but like in the video it only changes the top item and replaces it when trying to add a different item.

Forms!OrderF!OrderDetailF.Form!PartNumber = PartNumber

The code below shows me the record count. I wanted to use this in place of the not null syntax but it doesn't seem to work with an if statement.

msgbox Forms!OrderF!OrderDetailF.Form.Recordset.RecordCount

I've tried

    If Forms!OrderF!OrderDetailF.Form.Recordset.RecordCount > 0 Then
        DoCmd.GoToRecord acForm, OrderF!OrderDetailF.Form, acNewRec
    End If
    Forms!OrderF!OrderDetailF.Form!PartNumber = Me.PartNumber

IT gives me variable not defined error. How should I set the variable(s) or improve this?

I would use an INSERT INTO statement then requery the subform.

I finally figured it out. This code worked like how I wanted to:

    Forms("OrderF").OrderDetailF.Form.Recordset.AddNew
    Forms("OrderF").OrderDetailF.Form.PartNumber = PartNumber

It took me a lot longer than I'd like to admit. I kept trying to make the code below work (in different ways) or the docmd.gotorecord stuff. I can't tell sometimes when to use quotes or parentheses.

    Forms!OrderF!OrderDetailF.Form!PartNumber = PartNumber

Appreciate the feedback. The multiple options were helpful with different ideas.