This was tested on a Nexus 5 running CyanogenMod 13.0 (Android 6.0.1). This probably doesn't work on stock firmware.
On device:
Enable Debug Console (this is set in the Developer options)
On computer:
adb shell
logcat | grep "length =6"
On device:
Dial *#*#3282#*#*
(*#*#DATA#*#*)
Tap LTE, then Edit
On computer, make note of the logcat output:
LG_SVC_ITEMS: length =6 responseBytes=[B@abcdefg itemValue=######
Look for the number after the itemValue= part. This is your device's specific MSL code.
On device:
Enter the MSL!