The Fortmatic SDK offers it's own method for showing users just their Fortmatic wallet's public address.
// Initialize providerimport Fortmatic from 'fortmatic';import Web3 from 'web3';​const fm = new Fortmatic('YOUR_API_KEY');window.web3 = new Web3(fm.getProvider());​fm.user.deposit();
Users will be prompted to login into their Fortmatic account, after which they'll see their public address and balance: