Took some experimentation

Because I’m fairly inexperienced at managing Nagios, I had to hunt around to figure everything out.

Hopefully this will be helpful to others:

I added the command to objects/commands.cfg
I added the service to servers/services.cfg
I added the command to etc/nrpe.cfg on both the Nagios Server and the target Host

I added a line to nrpe/common.cfg on both servers of:
command[memory]=/usr/local/nagios/libexec/check_mem $ARG1$ $ARG2$

I restarted nrpe (xinetd) on the target host(s) with:
sudo systemctl restart xinetd

I restarted nagios on the nagios server
sudo systemctl restart nagios