adding custom action item menu in SharePoint online Modern team sites

Hey there, Deshani Perera! Let's get you started with #SPFx dev:

You can use Visual Studio, or your own custom development environment to build SharePoint client-side web parts. You can use a Mac, PC, or Linux.

Use this guide to help setup your environment:

https://docs.microsoft.com/en-us/sharepoint/dev/spfx/set-up-your-development-environment

Watch this video:

https://www.youtube.com/watch?v=_fxYexlUhe0&t=5s&list=PLR9nK3mnD-OXvSWvS2zglCzz4iplhVrKq&index=1

If you get a chance to attend the FREE all day Microsoft Office Dev Bootcamp, I highly recommend it!

Hope this helps!