There are a few scenarios that may make it necessary to image a Mac via Target Disk Mode.
1- If you have a Mac with T2 chip and do not have the admin password or authorization to change the secure boot settings on the Mac
2- The Mac has hardware damage, such as cracked screen, broken keyboard, or faulty USB port
3- The Mac model is not supported by the version of MacQuisition being used
For the third scenario, the prohibitory symbol (circle with slash) shown in the image below is displayed by macOS when it detects an unsupported boot environment. Please see this article for more information.
Steps for imaging via Target Disk Mode
1- Place the source Mac in Target Disk Mode (TDM) by first holding down the option key to check if a Firmware Password is required. Then release the option key and hold down the 'T' key.
For the 2018/2019 models, you should see a Thunderbolt 
and a USB 
icon appear on the screen. If you are not familiar with Target Disk Mode, Apple has information here.
2- Attach either a Thunderbolt cable or a USB 3.0 cable to the source Mac.
3- Boot to MacQuisition on a host Mac that has either a Thunderbolt 3, a Thunderbolt 2, or a USB 3.0 port.
4- Attach the source Mac that is in Target Disk Mode to the host Mac that is booted to MacQuisition.
To support 2018/2019 models, the host Mac model needs to be 2012 or newer. If the host Mac has a USB-C port, we recommend using this Apple USB-C TB3 cable to attach the source Mac in TDM to the host Mac. If the host Mac has a Thunderbolt 2 port, we recommend using this Apple TB cable and Apple TB3 to TB2 adapter. While they should be stocked in Apple or Best Buy stores, if one of these cables is not available, a 3rd party Thunderbolt (USB-C 3.1 ) or USB-C data cable may work. If an adapter is needed for the source or host port, we recommend only using genuine Apple adapters.
NOTE: For best results, we recommend booting to the MacQuisition device attached to the host Mac. If it is not possible to boot to MacQuisition on the host Mac, make sure to use SoftBlock or a hardware write-blocker so the source Mac is write-protected. In order to detect the 2018/2019 models while the host Mac is running live, the host Mac will need to have macOS 10.13.6 or newer installed.