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.