Validate Purchase Request (Requisition) Supplier Field

Dear All,

We are using Oracle EBS 12.2.14.

In Purchasing module, when creating Purchase Request (Requisition), in supplier field, user can select from List of Values, also, he can edit this field, and user can add any value without validation with supplier LOV.

How to disable this feature, and force validation on this field, and no further steps should be taken before selecting existed supplier from the Supplier LOV?

so that only values listed can be selected.

For Example, in this screenshot, supplier XXYYZZ is not listed in the LOV:

But user can continue creating PR, adding this value, although this supplier does not have any existence in system yet: