Currently, Shift does not support the functionality to add a custom app, but there is a workaround that may be helpful to you (see below). Otherwise, you can browse the 700+ apps we support in the Apps Directory, or by visiting the Apps/Extensions tab in Settings.
Your app will appear in your sidebar with the Jira logo, and the color and
name of your choice.
Please note that Shift favors integrations built by our own team, this workaround is not perfect and may not have all the functionality of running your app outside of Shift.