cd /usr/local/directadmin/custombuild
./build update
./build set redis yes
./build redis
cd /usr/local/directadmin/scripts/custom
wget https://raw.githubusercontent.com/poralix/directadmin-utils/master/php/php-extension.sh -O php-extension.sh
chmod 750 php-extension.sh
./php-extension.sh

./php-extension.sh install redis
./php-extension.sh status redis
./php-extension.sh remove redis

Full

cd /usr/local/directadmin/custombuild
./build update
./build set redis yes
./build redis
./build set_php redis yes
./build php_redis

Cannot build redis, because you do not have it set in php_extensions.conf file.
cd /usr/local/directadmin/custombuild 
./build update
./build php_extensions
./build set_php redis yes

cd /home/username
sudo -u username mkdir .redis
chmod 770 .redis

sudo -u username mkdir .redis/db
chmod 770 .redis/db

sudo -u username touch .redis/redis.sock
chmod 770 .redis/redis.sock

touch /usr/local/directadmin/data/users/username/redis.conf
chown diradmin.username /usr/local/directadmin/data/users/username/redis.conf
chmod 640 /usr/local/directadmin/data/users/username/redis.conf

systemctl daemon-reload
systemctl enable redis@username
systemctl start redis@username