Why not start UBI on a small, voluntary scale, and demonstrate its success? Every one advocating for/supporting UBI could go to a poor neighbourhood / country, pick a poor person (or several), and start transferring them a portion of their income every month, in exchange for periodic check-ups on how they are doing. The collected data could be beneficial for studying the impact of UBI on people's behavior.

If anything, a small number of people will start to benefit from UBI right away. That number will grow as more people join the effort. Who knows - voluntary UBI might be enough to the point where we do not need any government action for a pretty good approximation to "Universal" in UBI.