If you need a S.A.M.E. code, NOAA does these by county.
The zip code of 56573 is part of the county of Otter Tail, Minnesota, assuming no typing errors are involved. Thus the S.A.M.E. code is 027111 . Several transmitters cover this region 162.400 (channel 1), 162.500 (channel 5), 162.550 (channel 7), and 162.525 (channel 6) . However, the best transmitter choice is probably Channel 5 (162.500 WNG680) from Fergus Falls. The second best choice for Perham, MN is Channel 1 (162.400 Detroit Lakes WXM64 but it is a low power transmitter). See the link to the coverage map below.
All S.A.M.E. codes are available here: http://www.weather.gov/nwr/indexnw.htm
. Click on your State and then look for your county. The Minnesota S.A.M.E. codes are listed here: http://www.weather.gov/nwr/CntyCov/nwrMN.htm
. To see the coverage zones for each of the transmitters, see http://www.weather.gov/nwr/Maps/PHP/minnesota.php
(the link to the coverage map is also on the County S.A.M.E. code list page on the left hand side of the page.
Otter Tail is a large county and none of the transmitters seem to cover every inch. So you will need to try your weather radio and see which transmitter gives you the best reception of tests, watches and warnings.
I hope this helps.
(who has several weather radios in the house. They are in different areas and set to receive different transmitters. I live near a county line and the signal strength depends on where in the house you are. We tried using an external antenna. We received the signal from a transmitter from another state at the desired frequency and didn't receive the local warnings.)