See https://wordpress.stackexchange.com/q/341950 for details.

my-plugin.php

<?php
/**
 * Plugin Name: WooCommerce Custom Endpoint
 * Plugin URI: https://wordpress.stackexchange.com/q/341950
 * Description: Sample plugin which adds a custom WooCommerce (v3.6.4) endpoint named "Company Profile". =)
 * Version: 1.0
 * Author: Sally CJ
 * Author URI: https://wordpress.stackexchange.com/users/137402/sally-cj
 * Text Domain: my-plugin
 */

defined( 'ABSPATH' ) || exit;

// Add the company-profile endpoint.
add_filter( 'woocommerce_get_query_vars', function( $query_vars ){
	$query_vars[] = 'company-profile';
	return $query_vars;
} );

// Filter/change the endpoint title.
add_filter( 'the_title', function( $title ){
	if ( in_the_loop() ) {
		global $wp;
		if ( is_main_query() && is_page() &&
			isset( $wp->query_vars[ 'company-profile' ] ) ) {
			$title = 'Company Profile';
		}
	}
	return $title;
}, 11 );

// Add "Company Profile" to the "My Account" menu.
add_filter( 'woocommerce_account_menu_items', function( $items ){
	$items['company-profile'] = 'Company Profile';
	return $items;
} );

// Render the content for the "My Account" -> "Company Profile" section.
add_action( 'woocommerce_account_company-profile_endpoint', function(){
	$user_id = get_current_user_id();
	?>
		<form method="post" action="">

			<h3>Edit company profile</h3>

			<div class="company-profile-fields">
				<div>
					<?php woocommerce_form_field( 'company_id', array(
						'type'  => 'text',
						'label' => 'Company ID',
					), get_user_meta( $user_id, '_company_id', true ) ); ?>
				</div>

				<p>
					<button type="submit" class="button">Save company profile</button>
					<?php wp_nonce_field( 'edit_company_profile', 'edit-company-profile-nonce' ); ?>
					<input type="hidden" name="action" value="edit_company_profile" />
				</p>
			</div>

		</form>
	<?php
} );

// Save the company profile data.
add_action( 'template_redirect', function(){
	if ( empty( $_POST['edit-company-profile-nonce'] ) ) {
		return;
	}

	$nonce_value = wc_get_var( $_POST['edit-company-profile-nonce'] );
	if ( ! wp_verify_nonce( $nonce_value, 'edit_company_profile' ) ) {
		return;
	}

	if ( empty( $_POST['action'] ) || 'edit_company_profile' !== $_POST['action'] ) {
		return;
	}

	wc_nocache_headers();

	$user_id = get_current_user_id();

	$company_id = wc_clean( wp_unslash( $_POST[ 'company_id' ] ) );
	update_user_meta( $user_id, '_company_id', $company_id );

	wc_add_notice( 'Company profile data saved successfully.' ); // Just remove this if not needed..

	// These redirects both worked for me - no "headers already sent" error.
//	wp_safe_redirect( wc_get_endpoint_url( 'company-profile', '', wc_get_page_permalink( 'myaccount' ) ) );
	wp_safe_redirect( wc_get_page_permalink( 'myaccount' ) );    // Go back to the My Account page.

	exit;
} );