Auto fill invoice field from client field

What version of Invoice Ninja are you running? ie v4.5 / v5
v5

What environment are you running?
Self hosted

Have you searched existing issues/requests?
Yes

Issue

I have created a custom invoice field "Kundennummer" (client number) which is supposed to be visible in the invoice right below the invoice number.

Current behavior

Currently, I have to open the client in a new tab with each invoice and manually copy and paste the client number into the Kundennummer.
Expected/Ideal Behavior

There should be a way to auto-populate a custom field from another field: For example in my case, it would be great if the Invoice custom field "Kundennummer" gets automatically filled with the client number as soon as I select the client when creating the invoice.