If you’re using the Stacks plugin, you can turn the HTML into a partial for re-use within the project, or as an External for re-use across projects.
Let us know if you need any help with that.
If you don’t use Stacks, I’m afraid it’s not possible to store the HTML inside RapidWeaver and then use it across projects. You can re-use it within the project by storing it as a snippet though.
Alternatively, you can of course store it in the Notes app that comes with macOS (for example) and copy/paste it from there if you need it. This will work across projects.