Blank billing date in Milestone billing

Hi! We have a very simple milestone billing setup and  have configured milestone billing in ECC where a billing date is required with a default block.
What we would like is to have the billing date remains blank without the block so that users can enter the billing date when ready.

I can remove the block via configuration. However, I cannot seem to set the billing date as non-mandatory, i.e. able to leave it blank. Does anyone know of a solution? Do we need to set up a different control for FCode-OvervScrn but are we allowed to?

I cannot find any solution from SDN hence the post. Appreciate any advice on the above.

Cheers!
SF