I’m guessing you don’t have an AMS and can’t just print the lettering? Since the lettering appears to be on the top surface, you could reasonably do manual filament swaps to garner the same results.
All of the shirt inks I’m familiar with are heat cured, so I don’t have any suggestions for that, but I can maybe suggest a different method. You can create a 3d printed stencil of the lettering you want on the model and then use a paint marker to mark the lettering.