JSFiddle - React, Tailwind, and code Playground

HTML

<input type = "text" id = "a" value = "Miguel Ignacio" />
<input type = "text" id = "b" value = "Saavedra Campos" />
<input type = "text" id = "c" />

CSS

input{
    display: block;
}

JavaScript

var primero = document.getElementById("a").value,
    segundo = document.getElementById("b").value,
    tercero = document.getElementById("c"),
    primeraLetra = primero[0].toLowerCase(),
    primerApellido = segundo.substring(0, segundo.indexOf(" ")).toLowerCase(),
    final = primeraLetra + primerApellido;

tercero.value = final;