Adding tags with "#" to metadata, so you can add them with proposals instead of without

It sounds like you are editing your properties in Source Mode? If so, to use # in the tags property, you need to quote the tag:

---
tags:
  - "#example"
---

In Live Preview, the properties editor does this automatically if you type a # with your tag. (The properties editor is the default interface to properties in Live Preview. If you don’t see it, there is a setting to make it appear.)

If you want Obsidian to recognize tags in property fields other than tags, comment on this feature request: