Can not we import it into the custom field?

https://resmim.net/f/IhxCKW.png

Hello there,
I am creating a custom field. but it is forced to enter it from a different section, not from the private domain. I am doing bot in c #. and I need to enter the url in the custom field for the highlighted images. but it goes out of this private area. it does not allow you to add custom space at all. How can I overcome this annex?

Thank you