Yeah, it doesn't seem set up to help you discover retailers where you can actually use it. Their site does have a list of retail client partners (https://www.truefit.com/Partners/Retail-Clients) which gives a starting point.

I tried visiting several sites and got these results:

Works for me: Macy's, J.Crew, JCPenney

Doesn't work: Kohl's, Dockers, Lands' End, Ralph Lauren

In all cases, I picked a random (except for avoiding store brands) men's casual shirt and looked on the details page near where you select a size.

Looks like this is actually a B2B service that would sell it to existing retailers and/or tailers, not necessarily as something you yourself would sign up for.