Use the existing onboarding form and add an approval process to the onboarding tree.
The included Approval routine is called KD Individual Approval Create.
Request CE Form
The onboarding form includes a section to identify a manager. Use that information for the person to receive the approval.
Add the approval routine to your onboarding tree. Add logic to handle denials and approvals. Pass information or notes to notifications for the user and the new employee.
Extra - create a loop for multiple approvers. Add a team option ot the manager and pass to the approval.