I've done similar. Try making it 'Single Occurance' and Field Type = Input Only. Also check 'mandatory' and will need to fill it before they leave, if you want. Then, use the command 'input' to get to it on start up. I used this code
Code:
form.1:
init.form:
input("my.single.occ.variable")
execute(modify.set) |* takes them to modify the first occ