The easiest is to use the Remarks field, available for all openings - at least with the Properties available in the USA library. Presumably the FR version is similar.
The out-of-the-box USA 26 template (sample) schedules for Windows and Doors already includes the Remarks field. Here is where one would fill in the custom text in the WIndow/Door Settings dialog... and the next screenshot shows where you would select it as a scheduled field in the schedule customization dialog.
Note, the fields used in the "old days" - Custom 1, Custom 2, etc or User Def 1, User Def 2, etc are no longer part of the objects - gone since Properties were introduced some years ago (in case anyone searches the forums and finds old posts referring to them). The window/door objects themselves, at least in the USA library, do have a "Notes" field - available in the settings dialog - and to schedule by going to the library parameter section of the parameter selection list. But, I think the Remarks Property would be an easier choice...and of course, it is easy to add more properties if you need more than one custom field.
One of the forum moderators • AC 26 USA and earlier • macOS Ventura 13.2.1, iMac Pro 10-Core, Radeon Pro Vega 64