You need python-openstackclient

apt -y install python-pip3
pip3 python-openstackclient

You need to get the openrc.sh authentification file from the Public Cloud console

Then you can type (you will be prompted with your password)

source openrc.sh

openstack ec2 credentials create

You will see a table with the specific credentials for the S3 API

ACCESS_KEY = access in the table
SECRET_KEY = secret in the table
URL = https://storage.<region>.cloud.ovh.net (<region> could be rbx gra sbg bhs …)