How to indicate to user that a field is not optional, but that a blank value is considered valid input?

Question

We have forms in our application with a number of strictly required fields (marked with red asterisk, will fail validation if left blank). There is also one field that can be submitted as blank - this field is not marked as required so that validation will let a blank value pass. However, we want to indicate to users that this is not just an 'optional' field - it's a required piece of data, but a blank string is an acceptable option.

What we have found is that just leaving that field optional results in users skimming over and skipping it - they're just taking the shortest path to complete the form. We want leaving that field blank to be something they consider and make a conscious choice about.

What is a good way to indicate this to the user? I have considered something like putting in a truly required toggle switch that would open the input only if they mark it as 'yes' but that would also allow them to just skip over if they didn't care.

This is a mobile web page that is only used by internal employees who will become familiar with the UX, if that makes any difference.

In case it is useful to clarify why this is needed - users are entering data from physical labels. Some labels have this value on them, others do not. If the label has this value, we need the user to enter that field, but we don't know up front whether or not any particular label will have that value, so we can't make the form strictly require the value. The current system results in users not providing that value even when it is on the physical label because they view it as not required.

Answer 1

You could just utilize a paired checkbox with the field. Checking the box indicates that the user acknowledged the field and intentionally wanted to leave it blank.

This way a user cannot submit the form unless they either input data or check the box. May need to include some helper text so that the user isn't confused.

Answer 2

If you really need the data to be entered - make the input as required.

As your users are 'internal employees who will become familiar with the UX' they could be instructed that if label has no data on it they should enter predetermined value, e.g. 'N/A'.

You will be able to validate value entered and implement some application logic, e.g. to notify user look more thoroughly or ask 'Are you sure?' question and so on. Also this makes employee more responsible because in the future no one can tell that no entered data because just missed this field.

Also you may use hint like in Noveltee's answer.

Answer 3

You could handle this in one of 2 ways:

Option 1: ask if they have that information first

Do you know the colour of a unicorn?
( ) Yes
( ) No

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if yes ask them for that information

What's the colour of a unicorn?
[                 ]

[Continue]

Read more about this pattern in handling optional fields

Option 2: put ‘if applicable’ or ‘if you have it’ in brackets

Full name
[         ]

Colour of unicorn (if you have it)
[         ]

[Continue]