You would either have to use Vinyl (Cut by a plotter), Plastisol Transfers (Silkscreened onto a transfer sheet), or an Opaque transfer (Which can be ptinted on). I also believe that a DTG (Direct To Garment) machine can print white onto a shirt. I know they use it as a base.
I have tried heat pressing NUMBERS using unprinted Laser1 Opaque transfer, cut on a CAD cutter/plotter. A little difficult on small individually cut letterring though since you have to use a carrier tape, if not, you have to individually lay down each letters/numbers on the shirt. I think this would work on most opaque transfers including inkjet opaque. I would prefer the thicker opaque transfers for this job.
I think there is also white vinyl transfers and are usually the first choice of members here.
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