No available values in Report Builder parameter

In a Report builder report I have runtime prompt filter on a field.
But I am not able to see the available values in the dropdown for this field.
My filter is on a "New Field" with the formula :
IF(LENGTH(FieldName)>0,FieldName,("Blank"))

However if my filter is on the FieldName directly and not on the New Field with the above formula I do get the available values.
Is there any way I can get the available values in a dropdown for the New Field?

In Model Designer, add the "New Field" formula as a new attribute to the entity. Then set ValueSelection of the attribute to be dropdown.|||

True any field which is present in the Model can be shown in the dropdown.

I was exploring if a new formula field that is not actually present in the model, might also have 'Available values'.Thanks for your response.

sql