You have to use Smart Styles to define barcode text fields. Create or maintain a smart style to attach to your form.
There you create a Character format,
Creating Character Formats
You use character formats to assign special output attributes to sections of texts or character
strings within a paragraph. You must choose a different name for each character format.
For a character format, you can choose among the following attributes:
o Superscripting/subscripting the character string
o Font attributes (font family, font size, bold/italic, underlined, color)
You maintain barcodes in transaction SE73. If you want to display a field or a
number chain as barcode, choose the Standard settings tab. In the Name field select
the desired barcode key. After activating the Smart Style, assign the character format
to the field or character string in the PC Editor.
5. In change mode of the Smart Style select the Character formats node and choose Create.
6. In the Character format field enter a two-character character key.
7. Select the desired attributes on the individual tab pages.
8. Choose Activate.