I believe only Cisco-signed applications can be hosted on the bootflash.
Is there any way to sign 3rd party application?
Or is a USB SSD storage mandatory for them?
[Restrictions for Application Hosting]
> In Cisco IOS XE Amsterdam 17.3.3 and later releases, application hosting is enabled on the bootflash, however, only Cisco-signed applications are hosted.
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/prog/configuration/172/b_172_programmability_cg/application_hosting.html