Hi! We're still learning all of these things ourselves, so do take this with a grain of salt - we promoted the demo organically by doing a simultaneous post on as many relevant platform/ social media as possible, joining an event during this period or announcing the game during the event helps get the word out too! In our case it's really fortunate that the timing was perfect, as we released the demo and showcased it in Game Connection & GDC all in the same week :)
All in all - don't be afraid to put your work out there! it's always better to get harsh & productive feedback from the few that does play the game (especially in early stages), in comparison to waiting for the game to be 'perfect' and risk not having any input from anyone at all. So just go for it!