Custom value for custom feed atribute

Hello.

How can I create a feed in which certain custom attributes can be given a custom value?
For example, I created a custom feed and as a custom attribute I have – number of delivery days – and I have not customized something like that in my products, and I want all the products in the feed to have the value of 2, for example. I tried with suffix and prefix but it leaves me blank. Attached the images.

Thank you very much