Is there a function can Send HTTP Request? Want to call web service in Sail. ...

Yes, you need to deploy SendHttpRequest plug-in available under Shared component. It'd provide you with function called sendHttpRequest() that you can then use.

This is a plugin which can't be used in the Sail form.

The expression functions included in this plugin can be used on a SAIL form. If the HTTP functions don't meet your use case, you can also look at the webservicequery() OOTB expression for making WS calls on a form - forum.appian.com/.../Scripting_Functions.html