field for Paypal to see

What field should I add to the form that will allow me to enter specific data (ie. 200130-01*) that will pull over and be seen on the Paypal report, yet not visible to the user? 

If there is such a field, is there a specific place in the form where it should be placed?

I presume the specific data does not change?

If it is only a fixed input value, then that is possible. Here's a quick step by step:

1. Add a simple text box, set the default value on the specific data you want to pass. Right click and hide that field.

2. If you are using the standard Paypal integration. Click right on it and choose "Edit Properties".

3. In the properties window, you can set the "Custom Field" and assign it to the text box.

4. In my merchant test account it shows like this.

I checked my seller account but that custom data is not there. But to make sure, I suggest making some tests in your end.

I hope that helps.