An unbound field is unbound. It's not going to retain a value. Calculate the value of the field and put the calculation in the Control Source of the field.
In that case, you might consider using code to prompt the user what information they want to enter in the textbox as soon as the report opens. That should keep the information displayed as long as the report is open, but they won't be able to change it again, unless you add a button to do that.
Why are you typing data into a report? Why is the data not coming from a table? If it is ONE field, add an unbound control to the form that opens the report and reference that: