TurboTax uses a list of Locality ID Codes by State, Montana is not listed as a supported locality code so you would enter "XXX" in the "Locality" box.
Here is the list of Locality ID Codes supported by TurboTax
Kentucky Locality Codes
LEX - Lexington
LOU - Louisville
Michigan Locality Codes
ALB - Albion
BCK - Battle Creek
BRR - Big Rapids
DET - Detroit
FLT - Flint
GRR - Grand Rapids
GRA - Grayling
HAM - Hamtramck
HPK - Highland Park
HUD - Hudson
ION - Ionia
JAC - Jackson
LNS - Lansing
LPR - Lapeer
MHT - Muskegon Heights
MKG - Muskegon
PNT - Pontiac
PHN - Port Huron
POR - Portland
SAG - Saginaw
SPR - Springfield
WAL - Walker
Missouri Locality Codes
SL - St Louis
KC - Kansas City
New York Locality Codes
NYC - New York City, New York
YON - Yonkers, New York
Ohio City Codes
MR - RITA Municipal Tax
SDR - (four digit school district)
CCA - CCA Municipal Tax
ECC - CCA City Tax Form, Employment Tax Estimate
RCC - CCA City Tax Form, Residence Tax Estimate
Oregon Locality Codes
MUL - Multnomah, Oregon
All Other Localities
XXX - If your locality is not listed, or if this payment is for two or more localities
