Setting ethernet parameters from an string ESP8266

Hello, i am writing a wifi webserver for user setting ethernet parameters. I receive a string for each parameter. How can I set this parameters from this string? Example bellow simplified.

String buf_config = "";

server.on("/conf_ip", handle_ip);

IPAddress staticIP(buf_config);   // doesn´t work

void handle_ip()
{
buf_config = server.read();
}

//*******************
// I tried:

char buf_test[15]  

buf_config.toCharArray (buf_test,15)
IPAddress staticIP(buf_test);   // doesn´t work

Does this tool not satisfy your need:

https://www.digikey.com/en/maker/blogs/2017/configure-your-esp8266-wifi-module-with-new-wifi-manager