Horizontal Opt-in Form in Genesis using eNews Extended plugin and Flexbox. https://sridharkatakam.com/horizontal-opt-form-genesis-using-enews-extended-plugin-flexbox/

functions.php

// Register Horizontal Opt-in widget area
genesis_register_widget_area(
	array(
		'id'          => 'horizontal-opt-in',
		'name'        => __( 'Horizontal Opt-in', 'my-theme-text-domain' ),
		'description' => __( 'This is the horizontal opt-in section.', 'my-theme-text-domain' ),
	)
);

// Display Horizontal Opt-in widget area below header
add_action( 'genesis_after_header', 'sk_horizontal_optin' );
function sk_horizontal_optin() {
	genesis_widget_area( 'horizontal-opt-in', array(
		'before'	=> '<div class="horizontal-opt-in widget-area"><div class="wrap">',
		'after'		=> '</div></div>',
	) );
}

style.css

/* Horizontal Opt-in
-------------------------------------------- */

.horizontal-opt-in {
	background-color: #50c5b5;
	color: #fff;
	padding: 20px;
}

.horizontal-opt-in .enews {
	display: -webkit-box;
	display: -webkit-flex;
	display: -ms-flexbox;
	display: flex;
	-webkit-box-pack: center;
	-webkit-justify-content: center;
	    -ms-flex-pack: center;
	        justify-content: center;
	-webkit-box-align: center;
	-webkit-align-items: center;
	    -ms-flex-align: center;
	        align-items: center;
	max-width: 1000px;
	margin: 0 auto;
}

.horizontal-opt-in .widget-title {
	margin-bottom: 0;
	font-size: 20px;
	margin-right: 20px;
}

.horizontal-opt-in form {
	display: -webkit-box;
	display: -webkit-flex;
	display: -ms-flexbox;
	display: flex;
	-webkit-box-flex: 1;
	-webkit-flex: 1;
	    -ms-flex: 1;
	        flex: 1; /* grow this item to fill the available space */
}

.horizontal-opt-in .enews-widget input {
	margin-bottom: 0;
	width: auto;
	margin-right: 10px;
	padding: 10px;
	line-height: 1;
	font-size: 15px;
	border: none;
	-webkit-box-flex: 3;
	-webkit-flex: 3;
	    -ms-flex: 3;
	        flex: 3;
}

.horizontal-opt-in .enews-widget input[type="submit"] {
	margin-right: 0;
	padding: 10px 50px;
	background-color: transparent;
	border: 2px solid #fff;
	-webkit-box-flex: 1;
	-webkit-flex: 1;
	    -ms-flex: 1;
	        flex: 1;
}

.horizontal-opt-in .enews-widget input[type="submit"]:hover {
	background-color: #fff;
	color: #333;
}

@media only screen and (max-width: 860px) {

	.horizontal-opt-in .enews {
		-webkit-box-orient: vertical;
		-webkit-box-direction: normal;
		-webkit-flex-direction: column;
		    -ms-flex-direction: column;
		        flex-direction: column;
	}

	.horizontal-opt-in .widget-title {
		margin-bottom: 20px;
	}

	.horizontal-opt-in form {
		width: 100%;
	}

}

@media only screen and (max-width: 620px) {

	.horizontal-opt-in form {
		-webkit-box-orient: vertical;
		-webkit-box-direction: normal;
		-webkit-flex-direction: column;
		    -ms-flex-direction: column;
		        flex-direction: column;
		width: 100%;
	}

	.horizontal-opt-in .enews-widget input {
		margin-right: 0;
		margin-bottom: 20px;
	}

	.horizontal-opt-in .enews-widget input:last-child {
		margin-bottom: 10px;
	}

}