I did some reverse engineering - here is the bash, for anyone curious:
DOMAIN="mysuper-domain.com"
TOKEN="bla-bla" #cloudron's token
cd /var/db/acme/certs/*.${DOMAIN}_ecc
CERT=$(awk 'NF {printf "%s\\n", $0}' fullchain.cer)
KEY=$(awk 'NF {printf "%s\\n", $0}' "*.${DOMAIN}.key")
curl -X POST "https://my.${DOMAIN}/api/v1/domains/${DOMAIN}/config" \
-H "Authorization: Bearer $TOKEN" \
-H "Content-Type: application/json" \
-d "{
\"zoneName\": \"${DOMAIN}\",
\"provider\": \"manual\",
\"config\": {},
\"tlsConfig\": { \"provider\": \"fallback\" },
\"fallbackCertificate\": { \"cert\": \"${CERT}\", \"key\": \"${KEY}\" }
}"
verify with openssl s_client -showcerts -connect IP:443 -servername my.${DOMAIN}.net | less