certificate of coverage to prove exemption from self employment tax – US citizen?

I am doing my US taxes for the first time since starting a side business, I also have a salaried job.

I need to fill out a schedule SE, but I am exempt from self-employment taxes due to the totalization agreement. Apparently I need a certificate of coverage in Japan to prove this? Where would I get this? Or can I just make my own document stating I am exempt?

Edit: Since I get shakai hoken / pension through my regular employer I should ask my employer for this certificate?

Everything was going smoothly and I was feeling good about figuring it all out until I got to this point..