It may be harder to do that now.

One of the changes USPS made in recent years is that they are securing mail forwarding. That type of arrangement may not be possible.

It let me sign up. I had to pay $1.25 to "verify my identity" (?) as well as provide my phone number and email address. I believe I managed to avoid opting in to the initial spam offerings but I will of course still get a flood of "you just moved" junk mail, spam email, and phone calls for the next few months.