leaky-paywall-custom-field-subscriber-form.php

<?php 

// add the custom field
add_action( 'update_leaky_paywall_subscriber_form', 'testmag_add_company_field' );

function testmag_add_company_field( $user_id ) {

    $company = get_user_meta( $user_id, '_company', true );

    ?>
		<p>
			<label>Company</label><br>
	  		<input type="text" class="regular-text" value="<?php echo esc_attr( $company ); ?>" name="company">
		</p>

    <?php 
}

// save the custom field when the form is submitted
add_action( 'update_leaky_paywall_subscriber', 'testmag_save_company_field' );

function testmag_save_company_field(  $user_id  ) {

    if ( isset( $_POST['company']) ) {
        update_user_meta( $user_id, '_company', sanitize_text_field( $_POST['company'] ) );
    }

}