Under "System > Trust > Certificates" for GUI. For automated deployment, consider this link, reproduced below. Notably:
opnsense:
  cert:
    refid: (hex)
    descr: (cleartext)
    cert: (PEM + base64)
    prv: (PEM + base64)
  system:
    webgui:
      ssl-certref: (hex)
Sample script for automated deployment 
Certificate copied to /root/MyCert.crt and key to /root/MyCert.key:
#!/bin/sh
 
UNIQID=`/usr/local/bin/php -r "echo uniqid();"`
DATE=`date +%Y%m%d`
CRT="/root/MyCert.crt"
PRV="/root/MyCert.key"
FIRSTLN=`grep -m1 -n "<cert>" /conf/config.xml | cut -d ":" -f 1`
DESCR="My Certificate $DATE"
 
if test -f $CRT; then
  mkdir /root/certtemp
  cd /root/certtemp
  CRTE=`cat $CRT | /usr/bin/openssl base64`
  PRVE=`cat $PRV | /usr/bin/openssl base64`
  DESC="<descr>$DESCR</descr>"
  CERT=`echo "<crt>$CRTE</crt>" | tr -d '\n'`
  PRIV=`echo "<prv>$PRVE</prv>" | tr -d '\n'`
  REFID="<refid>$UNIQID</refid>"
  PATERN="\\n    $REFID\\n    $DESC\\n    $CERT\\n    $PRIV\\n  <\\/cert>\\n"
 
  echo "    $REFID" > temp.txt
  echo "    $DESC" >> temp.txt
  echo "    $CERT" >> temp.txt
  echo "    $PRIV" >> temp.txt
  echo "  </cert>" >> temp.txt
  echo "  <cert>" >> temp.txt
  cp /conf/config.xml config.xml
  sed "${FIRSTLN}r temp.txt" config.xml > config.xml.tmp
  sed "s@<ssl-certref>.*</ssl-certref>@<ssl-certref>${UNIQID}</ssl-certref>@" \
      config.xml.tmp > config.xml.new
  cp $CRT cert.pem
  cat $PRV >> cert.pem
 
  cp /conf/config.xml /conf/config.xml.$DATE
  cp config.xml.new /conf/config.xml
  cp /var/etc/cert.pem /var/etc/cert.pem.$DATE
  cp cert.pem /var/etc/cert.pem
 
  /usr/local/etc/rc.restart_webgui
 
  # Cleanup
  cd /root
  rm -rf certtemp/
  rm $CRT
  rm $PRV
 
  echo "New certificate $DESCR installed."
else
  echo "Nothing to do. Exiting."
fi
 
Manual restart of the web GUI using:
root:~# /usr/local/sbin/pluginctl webgui restart
Install vim:
root:~# pkg install vim
root:~# vim /var/unbound/host_entries.conf