Graham ebaze There is no 'sms' sending app on xtgem there is a url scheme but support may be a bit patchy on modern browsers the link takes the form the other options are possibly online sms widgets but most of those are 'premium' or 'ad powered' and some may cause spam messages if the service sells lists of numbers to advertisers
It gets rather complicated because of course apple cannot decide how they want to do it and there are several different url schemes depending on which version of IOS the device uses see http://blog.julianklotz.de/the-sms-uri-scheme/