This guide shows an old UI of XO, but you can always set the HA for the VM in its Advanced Tab.
HA at the pool level might be enabled using the command displayed in the blog post.
So far enabling HA on the pool level isn't exposed in XO because HA is causing more issues than it solves in general. If you decide to enable it on the CLI, it means that you probably understand what you do.
If you don't know where to find a SR UUID, using HA might expose you to VERY bad experience. BTW, you can find it in SR view, click on the SR you want to use as HA SR, you'll see the UUID just beneath the name.
As someone said in this forum few weeks ago:
Everybody wants HA but not everyone needs it