My custom page is redirected to a dynamics 365 form to create a record with an lookup field, the lookup field is not working

Question

I have two custom entities in D365, Purchase Requests and PR Lines.

My Custom Page is designed like this and is embedded in my MDA.

When I click the (+) button of Purchase Request Line section, it will redirect to the Main Form of Purchase Request Line to create a record, the Purchase Request should be auto populated based on the Purchase Request No. of the selected item under Purchase Request section, but that field is not being populated.

I tried to auto-populate non-lookup fields, it works, for Item Description, but not for the lookup field Purchase Request. This is a screenshot when the (+) icon is selected and navigated to main form.

This is my formula on select of the (+) Icon of Purchase Request Line:

Navigate(
Patch (
Defaults('PR Lines'),
{
'Purchase Request': LookUp(
'Purchase Requests',
'Purchase Request' = PurchaseRequests.Selected.'Purchase Request'
), 'Item Description': "Tst Description"
}
)
)

Custom Page in MDA

OnClick of Add Icon of Purchase Request Line

Edit Mode of my Custom Page