0365 Sharepoint permissions needed for a user to upload apps and use them in their site

Hi all, 

I have a Microsoft 365 with share point question. O 365 Users have the share point administrator role, they can create their web sites. There is a need for some of the users to develop their own web parts and use them into their websites.  They have the following roles applied to their users - SharePoint Administrator, Application Developer, they are site administrators.  

Currently those roles do not allow them to upload apps to their website. They can request new apps from the store, someone else have to approve them. My questions is what permissions, roles i need to  assign to the users for them to be able to upload and install apps (web parts) on their behalf.

Thank you for your time.