To make Square recognize it: set the item's SKU in your Square item library to exactly this code. Test with More → Settings → Hardware → Barcode scanner before the market.
Code 39 accepts A–Z, 0–9, space, and - . $ / + %. Code 128 accepts any printable character and prints about 25% narrower. Both uppercase automatically, so switching format never changes the value Square receives. Captions are print-only and aren't encoded.

Before using real label stock: print one test page on plain paper, hold it behind the label sheet against a light, and check every corner. In Chrome's print dialog set Scale to 100% (not "Fit to page"), Margins to None, and turn off headers and footers. Those are the usual cause of misalignment.
For plain sticker paper you cut yourself, leave Cut guides on and any layout will do.

Lot stickers print one per sticker, like a sticker roll, at the size above. The same 100% scale and no-margins settings apply. Use the number of units you filled from the batch as the copies.

Multiple item scan card: prints one page with every code laid out in a grid, each with its caption. Good for pottery price tiers: print once, keep it by the register, scan the card instead of the piece.
Captions are print-only and aren't encoded, so they can contain any characters. Each code still needs a matching item SKU in Square.