See: https://calderawp.com/doc/alternative-file-upload-directory/

caldera-forms-alternative-upload-handler.php

<?php

add_filter( 'caldera_forms_file_upload_handler', 'my_custom_uploader' );
function my_custom_uploader( $file, array  $args = null ){
	if( extension_loaded('ssh2' ) && function_exists( 'ssh2_connect' ) ) {

		$connection = ssh2_connect( YOUR_FTP_SERVER, 22 );
		ssh2_auth_password( $connection, YOUR_FTP_PASSWORD, YOUR_FTP_USERNAME );

		return ssh2_scp_send( $connection, $file['tmp_name'], basename( $file['tmp_name'] ), 0640 );

	}else{
		return Caldera_Forms_Files::upload( $file, $args );
	}
}