RE: How to auto populate contact details to lead values using a workflow?
Hi jimxmy,
There are two 1:N relationships between Contact and Lead entities and one N:1 relationship between Contact and Lead entities.
1:N

N:1

So i have a question ---contact details you need to populate to lead is from parent contact or child contact?
From your description, “selecting from the 'existing contact' to auto populate details”, existing contact means you need parent contact(lead_parent_contact relationship).

From your screenshot, you update ‘Originating lead’ records, which means contact details id from child contact(contact_originating_lead relationship).
what you have done is that update parent lead with new created child contacts.

In general, populating child(lead) records with value from parent(contact) records is common.
If so, you need create a workflow in the lead entity, then update the current lead with value from parent contact.

Choose dynamic value from parent contact.

If you must trigger the workflow in the parent(contact) record, you need adding workflow tool or using flow to achieve it because workflow can't update child records directly.
Dynamics-365-Workflow-Tools/Update Child Records.md at master · demianrasko/Dynamics-365-Workflow-Tools · GitHub
Update Child Records when Parent Record is updated using Microsoft Flow for D365 CE (cloudfronts.com)
Regards,
Leah Ju
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