Hello Together

I have the following table design:

Column “Invoice date” - Type “Date” - Example value: 2021-12-30

Column “Customer code” - Type “Name” - Example value “awc”.

Now I want to compose an invoice number from these two fields using a formula. Invoice date & “_” & {abbreviation}

My output will look like this: 2021-12-30T00:00:00.000Z_awc

But how the output should look like: 211230_awc

How can I solve this cleanly using a formula?