How to adblock opnsense is very easy and can be done by running this simple script directly on the machine.
#!/bin/sh
## Clean up any stale tempfile
echo "Removing old files..."
[ -f /tmp/hosts.working ] && rm -f /tmp/hosts.working
## Awk regex to be inverse-matched as whitelist
# - Project Wonderful does unobtrusive ads on a lot of webcomics
# - SolveMedia is needed for captchas on some websites
#whitelist='/(projectwonderful.com|api.solvemedia.com)/'
whitelist='/(api.solvemedia.com)/'
# All Blacklists
blacklist='http://winhelp2002.mvps.org/hosts.txt http://pgl.yoyo.org/as/serverlist.php?hostformat=hosts&showintro=0&mimetype=plaintext https://adaway.org/hosts.txt https://raw.githubusercontent.com/StevenBlack/hosts/master/hosts
https://mirror1.malwaredomains.com/files/justdomains http://sysctl.org/cameleon/hosts https://zeustracker.abuse.ch/blocklist.php?download=domainblocklist https://s3.amazonaws.com/lists.disconnect.me/simple_tracking.txt https://s3.amazonaws.com/lists.disconnect.me/simple_ad.txt
https://hosts-file.net/ad_servers.txt'
## Fetch all Blacklist Files
echo "Fetching Blacklists..."
for url in $blacklist; do
curl --silent $url >> "/tmp/hosts.working"
done
## Process Blacklist, Eliminiating Duplicates, Integrating Whitelist, and Converting to unbound format
echo "Processing Blacklist..."
awk -v whitelist="$whitelist" '$1 ~ /^127\.|^0\./ && $2 !~ whitelist {gsub("\r",""); print tolower($2)}' /tmp/hosts.working | sort | uniq | \
awk '{printf "server:\n", $1; printf "local-data: \"%s A 0.0.0.0\"\n", $1}' > /var/unbound/ad-blacklist.conf
## Original Proccess CMD
#awk -v whitelist="$whitelist" '$1 ~ /^127\.|^0\./ && $2 !~ whitelist {gsub("\r",""); print tolower($2)}' /tmp/hosts.working | sort | uniq | \
#awk '{printf "local-zone: \"%s\" redirect\n", $1; printf "local-data: \"%s. A 0.0.0.0\"\n", $1}' > /etc/unbound/ad-blacklist.conf
# Make unbound reload config to activate the new blacklist
#echo "Restarting unbound..."
#exec /etc/init.d/unbound reload
# Clean up tempfile
echo "Cleaning Up..."
rm -f '/tmp/hosts.working'
echo "Done. Please Restart the DNS Resolver service from the WebUI."