In that case, setting these properties should do the trick. They define a new button called `save-then-new` (but feel free to pick your own button name), set a label for it, and we define a process that runs when this button is activated. It runs the standard `save-final` process, and then issues a new document id. Here I restricted this for a form with app name `my-app` and `my-form`.
<property as="xs:string" name="oxf.fr.resource.*.*.en.buttons.save-then-new">
Custom Save
</property>
<property as="xs:string" name="oxf.fr.detail.process.save-then-new.my-app.my-form">
save-final
then xf:dispatch(name = "fr-new-document", targetid = "fr-persistence-model")
</property>
<property as="xs:string" name="oxf.fr.detail.buttons.my-app.my-form">
save-then-new
</property>
I hope this helps,
-Erik