Mapping Visitor GUID to Contact via REST API 2.0 Form Submit

I'm trying to figure out the correct way to architect a REST API 2.0 form submission solution that links the first-party cookie web visitor GUID to the contact.

I found the following documentation in the Eloqua Developer Help Center:

https://docs.oracle.com/en/cloud/saas/marketing/eloqua-develop/Developers/RESTAPI/Tutorials/mapping-contacts-form-submit.htm

At the end, it states

The GUID hidden field value must be in GUID format "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx". To submit form data when you do not have a GUID value, use a value with all zeros for the field. For example: "00000000-0000-0000-0000-000000000000".

Now, I then found the following documentation in the Eloqua Help Center: