It's not particularly difficult to start a new city.

The government simply asks large companies to open offices/factories in the new city in exchange for tax breaks/subsidies. Or give funding to a university to open a satellite campus. All you need is a promise for like 20k people to initially move. Then the government builds roads and utility networks. Private developers will also build housing if given the right financial incentive.

The 20k people will automatically lead to the same number moving in due to cheap housing, or for creating every day businesses, hospitals, schools etc. Within a couple of years you can setup up a feedback loop where the population is growing at 5-10% every year. There is no need to force anyone to do anything. Financial incentives are enough.