I have recently made an "Astrobiology" proposal on Area 51 (which was my first proposal). When I stumbled apon the "Please community link to the community...Area 51 cannot find that community for you" text field I was very confused. I thought this text field could represent my SE site URL (because that would be the community I would be hosting) so I chose astrobiology.stackexchange.com for that field.
I later found out that a bunch of Area51 crawlers and/or moderators deleted my proposal because "astrobiology.stackexchange.com" seemed not to be valid for the "Area 51 cannot find that community" text field.
Soon after, I read through the Area 51 FAQ and found out that the people behind Area 51 want all of our proposals to have an "enthusiastic, committed group of expert users who check in regularly". I have not met in person or thought of any users, expert or not, who I can trust here on Area 51 or even in the entire StackExchange network. Do you want me to link a username? I'm pretty confused.
If the community URL is invalid, then what does this community text field represent? What should I type in next time I try to re-propose Astrobiology SE?