There are many way ways to do this - but the easiest is probably to use a fill layer for your metallic material with a mask based on your PNG text.
1. Add a fill layer and either drop in a metallic material from the shelf (or from Adobe 3D assets ) or make one .A metallic material will have the metallness channel set to 1 and for your purpose, make it shiny by using a low value for roughness (say 0.2 or less)
Turn on the height channel and raise the height to a positive value (this will raise the lettering)
2. Add a black mask to the fill layer which will hide it
3. Click on the mask and choose Add Paint. This will let you paint on the mask
4. Import your PNG lettering using Import Resources and set the use as Alpha
5. Add that Alpha as a stencil (drag and drop into the stencil slot) and position it
6. Brush with white - so that you are brushing through the stencil onto the mask to reveal the metallic letters