Please prefix your question with [BANanoVuetifyAD3] (then your question) This makes it easy to indentify such questions related to this library.
You are not providing information here sadly. The location of the appbar, is it in the pgIndex or other page? You want to hide / show the button from other pages or from pgIndex? You want to hide/show a button that resides in pgIndex from other pages?
Each component has a hidden property and this can be handled at runtime by calling .Hide/.Show or UpdateVisible
pgIndex mostly behaves a little different because it uses the VuetifyApp and not the VueComponent.
To Hide and Show any item there from any other page, use vuetify.ShowItem(componentname)