Snippet exapmle for creating a custom field in the edit site form

mainwp-custom-edit-site-field.php

add_action( 'mainwp_manage_sites_edit', 'myhook_mainwp_manage_sites_edit', 10, 1 );
function myhook_mainwp_manage_sites_edit( $website ) {
	$myvalue  = apply_filters( 'mainwp_getwebsiteoptions', false, $website, 'my_site_value' );
	?>
	<div class="ui grid field">
		<label class="six wide column middle aligned"><?php esc_html_e( 'My Value', 'mainwp' ); ?></label>
		<div class="ui six wide column" data-tooltip="<?php esc_attr_e( 'My Value', 'mainwp' ); ?>" data-inverted="" data-position="top left">
			<div class="ui left labeled input">
				<input type="text" id="mainwp_managesites_edit_my_value" name="mainwp_managesites_edit_my_value" value="<?php echo esc_html($myvalue); ?>"/>
			</div>
		</div>
	</div>
	<?php
}



add_action( 'mainwp_update_site', 'myhook_mainwp_update_site', 10, 1 );
function myhook_mainwp_update_site( $site_id ) {
	$myvalue = isset( $_POST['mainwp_managesites_edit_my_value'] ) ? sanitize_text_field( wp_unslash( $_POST['mainwp_managesites_edit_my_value'] ) ) : '';
	apply_filters( 'mainwp_updatewebsiteoptions', false, $site_id, 'my_site_value', $myvalue );	
}