If you just want standard Alphanumeric in a UCC128 barcode, the easiest way is to use the defaults (Subset B):
EDIT: encodes "21Hello" in the barcode
This will allow for alphanumerics, but will not pad out to 19 digits.
It looks like in U mode, Subset C is not only the default, but it is the only character option. The alpha characters are always stripped to allow for correct numeric padding.
Subset A is tricky to deal with because of the numeric pairs you have to give as data. To encode the string "21H", you have to enter ^FD>9181740^FS
Note I changed the ^FN to an ^FD to show the field with alphanumeric data. ^FN is used to specify a field in a format, not data.
Alphanumerics are possible in UCC-128 but mode U is not required. FN1 triggered by >8 is the switch that enables UCC-128. The following test ZPL code allows you to print and scan a UCC-128 barcode with either a start code of A (>9) or a start code of B (>:) containing alphanumeric characters:
^FT111,48^A0N,28,28^FH\^FDCode 128 B^FS
^FT83,230^A0N,28,28^FH\^FDCode 128 A^FS
UCC/EAN-128 bar codes always contain a special non-data character known as function 1 (FNC 1), which follows the start character of the bar code. It enables scanners and processing software to auto-discriminate between UCC/EAN-128 and other bar code symbologies, and subsequently only process relevant data.