What would you call the attracting of the initial customers?

I'll first answer in idealized terms. These initial customers should be early adopters. Early adopters don't need to be convinced. They would jump board, because you are offering a solution they have been looking forward to finding.

This is a very romanticized view of starting a business. And, IMHO, this is a formula to build businesses that appeal to VCs. In reality, starting a business begins with listening to potential partners. I don't like to call them customers, because they are not, at this stage. One should test if there are common problems. Validate that these are worth solving. Develop an MVP. Rinse and repeat.

Market research or product fit

validation? proof that there are people who actually want to use your product?

Customer development