you can do the opposite.
Create the field as required and set as not required when the record is created.
inside the onload script you need to add a javascript:
function FormOnLoad() {
if (Xrm.Page.ui.getFormType() == 1) {
// make the field not required when the form is on create
Xrm.Page.getAttribute("new_field").setRequiredLevel("none");
}
}
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