Edit: Prusa just announced that Prusaslicer 2.60 is going to get a text embossing feature. Not only will it allow for direct text editing, the text can also be made to stick to the surface of a model. Therefore, you won't need my workaround anymore. In case you can still apply them differently, I'll leave the files as they are.

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Until Prusa adds a text feature to Prusaslicer ;) you can use these letters for example as a negativ on your prints. Load the alphabet with the font you want to use (times new roman, calibri, arial) into prusaslicer. Split into objects so you can manipulate individual letters.