nginx-proxy-manager/src/backend/templates/_certificates.conf

{% if certificate and certificate_id > 0 -%}
{% if certificate.provider == "letsencrypt" %}
  # Let's Encrypt SSL
  include conf.d/include/letsencrypt-acme-challenge.conf;
  include conf.d/include/ssl-ciphers.conf;
  ssl_certificate /etc/letsencrypt/live/npm-{{ certificate_id }}/fullchain.pem;
  ssl_certificate_key /etc/letsencrypt/live/npm-{{ certificate_id }}/privkey.pem;
{% endif %}
  ssl_certificate /data/custom_ssl/npm-{{ certificate_id }}/fullchain.pem;
  ssl_certificate_key /data/custom_ssl/npm-{{ certificate_id }}/privkey.pem;
{% endif %}