Auto populate fields

I am using a form to add new records. The first field is populated using a combox that takes its values from another table. The value of this field will be the same in a series of enteries so I want to auto populate it based on the last record. I have added a line of code to set a varialble the first time the value is selected in the combobox and then tried to set the default value with the variable name in the properties window. However, I am not sure how to put a varaible name in the expression builder as it does not seem to recognise the name.