To send credits use the chat command "/send [steam64id] [amount]".
For example, to send 100 credits to steam64id XXXXXXXXXXXXXXXXX you'd type "/send XXXXXXXXXXXXXXXXX 100".".
Alternatively right click the person's avatar in chat and select "Send credits".
To find your steam64id you can use sites like steamid.io
Credits are obtained by depositing CS:GO skins. If you've used up the free 100 credits you'll need to make a deposit to get more..DO NOT contact support asking for credits.
To generate your own referral code please visit the affiliates page located here. affiliates.