In Ethereum, we have several ways to get airdrop, one way is just fill in our ethereum address in one form, another way is that you sand zero eth to a contract address and you can get some tokens back.
In CKB, the first way may not work, but the second way could be work. Over a period of time, once I have received your 100 CKB, I will send you 100 CKB + 100 UDT back. Users only pay a little gas fee.
As a project, I may send every live addresses on CKB an Open Transaction (About Open Transaction). You want my airdrop? Return my CKB.