Are you talking about the stakeholder/approver email?  Within the workflow, you can send a reminder email every yx] hour.

I want to send the email reminder on demand, i.e., right now.

@desktopsupprt1 I just wanted to add, the Onboarding Initiator can send a reminder email from the request, so long as it is still waiting for the Manager to do their part:

The real problem here is that the entire OnBoarding journey depends on the link in the email sent to the Reporting Manager.   If they are not available (and they forgot to set the delegation), the flow is stuck.   The only way is to - assuming you have access - go to the Sent Items in the mailbox associated with FS and forward a copy of that email to someone else.   

I stongly suggest that Freshworks re-visits and removes this dependency or provides a workaround within the platform itself. 

Will see what getting changed in the new Employee journal. But if you wish to get a change you need to create a Idea post.