Ask HN: Is it viable to raise money from your customers instead of investors?

I am working on a product for lawyers private offices. Its a free product so I cant have an upfront sale. Yet I am thinking instead of raising fund from professional investors, I might be able to raise the fund from my customer and give them equity. With this approach they would be spreading the word too and be religious advocates for it. Would it be a viable solution to do this throughout the lifecycle of an startup to only raise money from your customers?