Sometimes it's impossible to fill in all fields in deal/lead forms at once because you just don't have the required information yet.
Make particular fields Required starting with particular stages and fill in fields as you get the needed information.
How it works
For example, let's create Rejection reason field and make it Required for Deal lost stage.
Open a deal form > click Add field > Text.
Enter Field name and activate Required starting with stage option.
Select Deal lost stage only.
Done! Let's create a new deal.
Then try to change the deal status to Deal lost. Bitrix24 will ask you to enter the rejection reason.