If you google `PlantUML Gitlab` you should find some blog posts and docs, but if you are don't fancy reading this, try pasting this into a Gitlab Issue body

   ```plantuml
   Alice -> Bob: Authentication Request
   Bob --> Alice: Authentication Response
   
   Alice -> Bob: Another authentication Request
   Alice <-- Bob: Another authentication Response
   ```

I believe Github has similar support also.

See this for a whole lot more about PlantUML: https://plantuml.com/sequence-diagram