You would usually point the portal at a table (occurrence) where the entities to be selected are "at home", so there is only one record per entity instance.
More to the point: if you want to use a portal as a picker for a university, show records from a University table.
[...] inside portal tool i add drop down list to choose partner [...]
I'm not sure I see a drop down list there (nor do I see why you would need one ...)
I've used previously
When one problem now is to repeat
Is there a way when choosing the value of drop down list
after that the value Deleted
OK, I have to confess that I've no idea what you are saying...