Use a project action for a command you repeatedly run in that repository, such as starting development or checking a build. It is a shell operation, not an Agent prompt.
Define it where the project is configured
Open Settings → Projects, select the project, and edit Project Actions. Give the command a useful label. If it starts a development server, configure its preview behavior there.
Run against the intended checkout
Use the project-action icon menu in the header. The active session or worktree determines the working context; check it before running a destructive command. Inspect terminal output to see whether execution actually succeeded.
When output contains a recognized URL, the action can expose a preview. A remote desktop session may also use SSH forwarding; a phone does not create that tunnel itself.
If you want a reusable instruction for an Agent instead, create a command.