Open up a trigger and add/edit expression via modal popup. Change function to "Time to reach threshold estimated based on period T is NOT N", for example or one that has many parameters. See in source that there is same field paramtype. One for each function parameter. One of them is a <select> field, others are hidden. Thankfully user is not affected, since it submits the last value from <select>, but that code needs to be looked at. Also HTML ID's are identical for paramtype.