To add UpCloud as a server provider to enable easy server creation in Ploi, you will have to create a new member inside your account. They are called 'People' at the navigation of the UpCloud panel.
Add a new member bij pressing "+ Add", this screen will allow you to create a new user and specify some additional options.
Note: if you want to be extra secure, make sure you only allow this IP address for your member that you are creating: 22.214.171.124
After this has been done you can go back to Ploi.io and add it as a server provider in your profile. Just enter the username and password and you're all set!