boot from second disk

[Sergey Matveychuk]

Hi.

I set up FreeBSD 11.0 on first disk and next I added second disk and I 
want to do a mirror remotely. I followed instructions on 
https://www.freebsd.org/doc/handbook/geom-mirror.html
With one difference: I use GPT partitions (as default in the FreeBSD 
intaller):
# gpart show ada0
=>       40  351651808  ada0  GPT  (168G)
          40       1024     1  freebsd-boot  (512K)
        1064  343931904     2  freebsd-ufs  (164G)
   343932968    7718879     3  freebsd-swap  (3.7G)

And I stuck on "Restart the system, booting from ada1". I don't know how 
to do it remotely (IPMI is not available there).

I tried echo "1:ad(1,a)/boot/loader" > /boot.config

But it does not look work. What should be correct content of 
/boot.config for GPT?