Can I make a custom "text" field an automation in the request form?

In the request form I can get a custom field to work using a drop down, however I haven't been able to using a "text" custom field. 

When we ask the team a required question about there budget # for the print project they'd like completed, we'd like to not only make it required (that I can do), but also be an automation in our custom fields section at the top of the project. So it they manually type it into the brief, but once brief is submitted it auto populates in the creative brief, not as a field in the brief but as a custom field. 

Is that possible? If not, do you have any other recommendations?

We can't use a drop down for the budget as there was way to many budget numbers in our accounting system to have someone scroll through it to choose one. It's easier to type it in. However when we put it as only a question in the creative brief, they always side step it, even if it's required. It's one of the pieces of information that we always receive last and we're trying to remedy that.

Much appreciated, thank you.