Edit in JSFiddle

document.querySelector( 'button' ).addEventListener( 'focus', ( { relatedTarget } ) => {
	document.querySelector( '#relatedTarget' ).textContent = `#${ relatedTarget.id }`;
} );

<input type="text" id="input1" autofocus> <button>Button</button> <input type="text" id="input2">
<p id="relatedTarget"></p>