Short answer: Go to the places where your buyers already are. With no audience, your first users come mostly from direct outreach, communities, launch directories and search content. Expect the first 100 users to come from effort, not from a launch day.
Why having no audience is not fatal
An audience helps, but it is not required. Your first users need a clear reason to try your product and a way to find it. You can build both without followers.
Step 1: Define who the product is for
Write one sentence: "For [specific person] who struggles with [specific problem]." If you cannot finish it, you are not ready to look for users.
Step 2: Find 50 people who have the problem
Look where they talk about it:
Posts asking for tool recommendations or "alternatives to X."
Subreddits and forums for your niche.
Comment sections on posts about the problem.
Reviews that complain about existing tools.
Write their names and links in a simple sheet.
Step 3: Message them respectfully
Most cold messages fail because…