If this is FileMaker 13, you can set a Hide Object When property to keep the drop down list field from being visible until they have "Chosen a record in portal".
Is there not a way to write up a script for it? I am using Filemaker 12.
Yes there is, but since you are using an older version other methods would need to be used to make the drop down list appear and disappear.
just to avoid posdible confusion, can you describe exactly what you mean by "choose a record in a portal"? I can think of more than one set of user actions that fits that phrase.