This is the smallest useful WiFiManager application. It first tries the credentials the ESP platform already knows. If it cannot connect, it opens an access point named WiFiManager Basic with password example-pass.

Build and flash the selected esp8266 or esp32 environment.
Connect a phone or computer to WiFiManager Basic.
Open http://192.168.4.1/ if your captive-portal helper does not open it automatically.
Select a network and save it. The board joins that network and the next reboot reconnects without opening the portal.

The access-point password is only an example. Choose a unique, Wi-Fi-valid password for a real product.

See the shared example guide, getting started, and provisioning lifecycle.