System properties like this are added in the ignition.conf file, which is located in the data folder of your Ignition installation.
In the ignition.conf file you’ll want to find the Java Additional Parameters section, which on most 8.0 installs will look like this:
wrapper.java.additional.1=-Ddata.dir=data
#wrapper.java.additional.2=-Xdebug
#wrapper.java.additional.3=-Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=*:8000
You’ll want to add the additional properties underneath the -Ddata.dir property already in use, like so:
wrapper.java.additional.1=-Ddata.dir=data
wrapper.java.additional.2=-Dignition.http.session.cookie.same-site.enabled=true
wrapper.java.additional.3=-Dignition.http.session.cookie.same-site.value=Lax
#wrapper.java.additional.2=-Xdebug
#wrapper.java.additional.3=-Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=*:8000