Is the model UV mapped ?

The method you use is more or less the same whether it is or isn't, but if it is, you always have the option of modifying the texture (or copy) to add the decal directly.
The basic recipe is to use a Projected Texture node, with repeat set to "Once" in both directions, and adjust the offsets and scales to position the decal correctly.
The material your modifying must be in either "Object Standard" or "Object Parametric" mode to get the correct effect.

To get just the lettering, you'll need to create a mask image (or add an alpha channel).

The application of the decal can be done either as a modification to the node setup of the current material or as a new layer above the current material.