If the key is not named id_rsa it does not work

Arronical's answer is good for a one-time connection, but would be painful in the long term. In order to make ssh always use this key when connecting to this server, you can add this to your ~/.ssh/config (or create it if it does not exist):

Host vps718449.ovh.net
    IdentityFile ~/.ssh/id_rsa_admin

Then you can connect with just ssh [email protected] -p 9215. As an added bonus you can also configure the port:

Host vps718449.ovh.net
    IdentityFile ~/.ssh/id_rsa_admin
    Port 9215

Then you can connect with just ssh [email protected]. Even further:

Host vps718449.ovh.net vps
     Hostname vps718449.ovh.net
     User admin
     IdentityFile ~/.ssh/id_rsa_admin
     Port 9215

Then just ssh vps will work! Of course vps is just an example, you can use any other alias you want (for example if you have more than one VPS). See man ssh_config for a list of the options you can set there.

You need to use the -i option to your ssh command using the path to your key. The modified command will be:

ssh -i ~/.ssh/id_rsa_admin [email protected] -p 9215