Each WebSession has a unique ID you could use.
WebSession.Identifier
Once upon a time @Kem_Tekinay wrote a cross platform GUID function. I have an adjusted version that lives in a GUID module, so you can use it like this:
var s as String = GUID.Generate
Here is that module:
GUID.xojo_code - 5kb
This is cross platform for Mac, Windows, and Linux - so it will work for Web Apps.
Einhugur Util plugin also has a GenerateUUID(true)
function that will work for Web Apps. MBS has UUIDMBS.UUID.ValueFormattedString(false)
.