Adding Full Text Search via command line
I used the GUI version of setup to create a ConfigurationFile.ini that can be used to run the command-line setup to add the necessary Full Text Search components to an installation of SQL Server Core.  

The contents of the ini file, which would need to be modified for your scenario, include QUIET="True" which prevents setup.exe from displaying a Graphical User Interface.

Modify the below items for your needs, then save the file as ConfigurationFile.ini in a location where you can refer to it when running setup.exe on the core server.

;SQL Server 2012 Configuration File
[OPTIONS]

; Specifies a Setup work flow, like INSTALL, UNINSTALL, or UPGRADE. This is a required parameter. 
ACTION="Install"

; Detailed help for command line argument ENU has not been defined yet. 
ENU="True"

; Parameter that controls the user interface behavior. Valid values are Normal for the full UI,AutoAdvance for a simplied UI, and EnableUIOnServerCore for bypassing Server Core setup GUI block. 
UIMODE="Normal"

; Setup will not display any user interface. 
QUIET="True"

; Setup will display progress only, without any user interaction. 
QUIETSIMPLE="False"

; Specify whether SQL Server Setup should discover and include product updates. The valid values are True and False or 1 and 0. By default SQL Server Setup will include updates that are found. 
UpdateEnabled="False"

; Specifies features to install, uninstall, or upgrade. The list of top-level features include SQL, AS, RS, IS, MDS, and Tools. The SQL feature will install the Database Engine, Replication, Full-Text, and Data Quality Services (DQS) server. The Tools feature will install Management Tools, Books online components, SQL Server Data Tools, and other shared components. 
FEATURES=FULLTEXT

; Specify the location where SQL Server Setup will obtain product updates. The valid values are "MU" to search Microsoft Update, a valid folder path, a relative path such as .\MyUpdates or a UNC share. By default SQL Server Setup will search Microsoft Update or a Windows Update service through the Window Server Update Services. 
UpdateSource="MU"

; Displays the command line parameters usage 
HELP="False"

; Specifies that the detailed Setup log should be piped to the console. 
INDICATEPROGRESS="False"

; Specifies that Setup should install into WOW64. This command line argument is not supported on an IA64 or a 32-bit system. 
X86="False"

; Specify the root installation directory for shared components.  This directory remains unchanged after shared components are already installed. 
INSTALLSHAREDDIR="C:\Program Files\Microsoft SQL Server"

; Specify the root installation directory for the WOW64 shared components.  This directory remains unchanged after WOW64 shared components are already installed. 
INSTALLSHAREDWOWDIR="C:\Program Files (x86)\Microsoft SQL Server"

; Specify that SQL Server feature usage data can be collected and sent to Microsoft. Specify 1 or True to enable and 0 or False to disable this feature. 
SQMREPORTING="False"

; Specify if errors can be reported to Microsoft to improve future SQL Server releases. Specify 1 or True to enable and 0 or False to disable this feature. 
ERRORREPORTING="False"

; Specify the installation directory. 
INSTANCEDIR="C:\Program Files\Microsoft SQL Server"

; Add description of input argument FTSVCACCOUNT 
FTSVCACCOUNT="NT Service\MSSQLFDLauncher$INSTANCE"


An example command-line might be:

setup.exe /IACCEPTSQLSERVERLICENSETERMS /CONFIGURATIONFILE configurationfile.ini

See https://docs.microsoft.com/en-us/sql/database-engine/install-windows/install-sql-server-using-a-configuration-file for details about the configuration file.