Unfortunately, this could not be achieved when targeting ms.vss-work-web.work-item-form
.
We delivered the ability in the admin UI to configure which types
get an extension but we have no immediate plans to be similar logic
in the json manifest.
The reason also described as below:
We considered letting the extension author specify this in their JSON
manifest but that is too limiting since users will soon be able to
create custom work item types and an extension author would never know what those are.
Source Link
So it't not able to do the limitation as an extension developer, the users could do their customization in the process --work item type-- Layout page.
They could also do this by export the work item type, details please refer the Extensibility with the new work item form part in the link.
Besides, since you are doing the work item form customization, take a look at this tutorial Creating work item form extensions in Visual Studio Team Services which maybe helpful to you.