The location field for a telco circuit is calculated automatically from the location of the DMARC asset.
First you need to add DMARC as an asset specifying its location: 
Then under telco circuit editing page (or when you are creating new telco) select the DMARC:
And finally the location will be calculated for you:
DMARCs are treated as Assets and containers for Telco circuits in Device42. It is an actual physical object it is typically the drop point of a vendor to internal infrastructure network connection, could also be a point where back-end fiber changes to copper, etc.
