union card From Merriam-Webster:
1 : a card certifying personal membership in good standing in a labor
2 : something that resembles a union card especially in being
necessary for employment or in providing evidence of in-group status
Definition #2 is the one that answers the OP's question.
A made-up example of how it might be used:
I'm taking night classes to get my BA, because I'll never get a management job
unless I have my union card.
(This is an ironic example, because traditionally, union jobs were always non-management jobs.)
Peter Thiel, billionaire co-founder of PayPal is an extreme example of thinking of a college diploma as unnecessary, even harmful, as described in the Newsweek Article Peter Thiel Thinks You Should Skip College, and He’ll Even Pay You For Your Trouble. Thiel thinks today’s elite universities are overpriced relics and is offering fellowships to uniquely qualified teenagers so that they can pursue radical innovation that will benefit society. The catch: they must drop out of college.
The term is not meant to disparage blue-collar jobs. Indeed, such jobs can be more interesting and more rewarding and require just as much smarts and skills as white collar jobs. But, traditionally, a college diploma was the route to upward mobility, economically and socially, but is less so now, and now often comes with the burden of enormous student loans. I remember the term union card from my own college days, decades ago, when it was much less apt than it is today.