I am not 100%sure what you mean by by no platform, but I will answer as best I can.
If you have a website (again I am not sure if this is what you mean by no platform), you could have an email signup list.
What I see a lot of professionals do is give away something as an incentive like an ebook. If you have something that you can give away, people would need to enter their email address so it gets emailed to them and voila, you start building your list.
I also use social media and as I make connections, my mailing list builds fast.
I would be glad to schedule a call with you so I can share with you all of my strategies for building a list. I have 3 main strategies that I use to build my mailing list and can share them with you on a call.