That function is only available within the Ninox App for Mac, iPad or iPhone. At first you need a barcode font on you device. If you do not have one installed, please download and install a barcode font.
After that you can use that font in your print layouts or also in your input forms.
To use it in you print layouts please click into the field that shall be transformed into a barcode and choose the barcode font:

If you want to show the barcode also in your input form you can use an additional formula field and use a formula like this:

html(""<span style='font-family:New'>"" + Item + ""</span>"")"