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UKR is an experienced and helpful user; but I think he does not understand that there is another way to accomplish the same result ... a "way" that some users may prefer.
First: In my experience - and in the test I just ran using Q2020 R35.27 - an invoice payment received in a checking account (for example) and recorded as a transfer to a Customer Invoice account (with the payee name of an unpaid invoice), will be recorded in the Customer Invoice account as a (customer) payment (it will have "PMT" in the "Type" column in the Customer Invoice account (it will NOT have "CRED" in the "Type" column).
But, as SuperUser UKR notes, that transaction in the Customer Invoice account (created by the "transfer" from the account where the real-world payment was deposited) will not be linked to the invoice it is intended to pay.
Second: When dealing with one - or only a few - such unlinked PMT transactions; I believe the simplest way to match those unlinked PMT transactions to the correct Invoice, is to simply key the appropriate invoice number in the PMT transaction Invoice# field for each PMT transaction. That will cause Quicken to assign the Customer Invoice account PMT transaction to the Invoice with the same number, and change both the PMT and INVC transactions to be marked as "cleared" (to put a "c" in the cleared status field of each transaction).
[In the event there are many such unapplied PMT transactions in a Customer Invoice account, at some point it will likely be simpler to employ UKR's method ("create a $0.00 New Customer Payment transaction [in the Customer Invoice account]. In the data entry dialog form mark the field 'Apply existing credits' with a checkmark")]