This question is hard to answer without additional information about the purpose or what you/the user wants to achieve.
But if we take twitter or instagram as an example for user generated content, then hashtags are nothing different than labels. This is a well known and flexible system.
If the categories/labels are predefined, then i would propose to make suggestions for the generated content. Suggestions could be based on the text the user wrote or the user searches for tags and gets synonym or suggestion labels based on the search-term.