cd /var/www/vhosts/projectname
echo "export SECRET_KEY=xxxxxxxxxxx" >> .env
source venv/bin/activate
pip install python-dotenv

edit WSGI.py

import os
from dotenv import load_dotenv
project_folder = os.path.expanduser('/var/www/vhosts/projectname')
load_dotenv(os.path.join(project_folder, '.env'))

also in WSGI.py change dev to production

add to production.py

import os
SECRET_KEY = os.getenv("SECRET_KEY")