I just purchased a new Px4 and noticed that the slide has a crack in it. How do I get it repaired?

Cracked PX4 Slide

The crack that you are referencing as indicated in the image below is actually a machining cut in the slide.  This cut is a slide vent and is common in many semi-automatic pistols.  You will observe a similar cut in our 90-series pistols as well.  Your firearm is fine and safe to use.