By default, Certbot will request a certificate with RSA. If ECC is your preferred choice, we support the P-256 NIST curve. The following steps [1-3] is one approach to obtain it.

[1] Generate the key

root@acme# openssl ecparam -out server.key -name prime256v1 -genkey

[2] Generate the CSR

root@acme# openssl req -new -key server.key -out server.csr
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:
State or Province Name (full name) [Some-State]:
Locality Name (eg, city) []:
Organization Name (eg, company) [Internet Widgits Pty Ltd]:
Organizational Unit Name (eg, section) []:
Common Name (e.g. server FQDN or YOUR name) []: <YOUR_DOMAIN>
Email Address []:

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

[3] Apply for a certificate

root@acme# certbot certonly --webroot -w /var/www/example.com/public_html/ --csr server.csr --server 'https://api.buypass.com/acme/directory'