I don't particularly love the branding of growth hacker but let's take it as given that it usefully describes a set of skills.
Practical experience outweighs "credentials" by literally infinity. Portfolio pieces are extremely helpful. People who are incapable of doing any useful work abound in this and related fields, and employers suck at hiring, so case studies and testimonials are virtually the only sorting mechanisms with any value.
A bright engineer can learn to be dangerous with marketing in between 2 weeks and 6 months depending on the engineer and circumstances. (Some of the overlap of those two fields is commonly called growth hacking, but that isn't coextensive with the field. I am, personally, skeptical of the value-add of growth hackers who cannot, in fact, ship software when required to. My skepticism is not shared by at least some influential people who write about growth hacking frequently.)
If you can reproducibly print money on command with your skills, be they marketing/engineering or growth hacking, you should probably never work for free (working for "just shares" is working for free) and will probably find your specialty in absolutely stratospheric demand at the moment. Chargeout rates in the 5 figures per week are totally reasonable in the current environment.