How do I add custom buttons(CommandBarItems) on CommandBar in Modern View of SharePoint Online
3 Answers
Create a SharePoint Framework Extention.
More info here: https://docs.microsoft.com/en-us/sharepoint/dev/spfx/extensions/overview-extensions
Please check out the below given links for help:.
Add a button to Command Bar of a SharePoint online
Add CustomAction button in Ribbon of DocumentLibrary in SharePoint Online