Form question

Short question is can you populate a dropdown on a form from an aggregate list and pre select one of the values?

Long question with example:
I have a form lets call it "myform" located in a web block.  In the form lets say there is a user selection drop down.  The source list for this is a list from an aggregate in my prep for the web block.  So in the prep the last thing I do is have an assign where I assign a value to the variable of the dropdown in the form.  So myform.record.userselection gets a user id.  I am also doing this with a datetime in the form where in the prep I assign myform.record.datetime to be currdatetime().  However, when the page loads with the web block, instead of the values I told it to be, its just the "-" and if I click on it the drop down list and the date time says MM-DD-YYYY HH:mm:SS.  Is there a way to have the drop down list and a datetime on a form where you prepopulate it from the prep?
Quick update to this.  If I have the field on the form point to a local variable instead of the form record it will work as I want, however then I have to handle transferring the variable when I load/save.  I would like to find a way to keep the variable pointing to the form record but still be able to pre load it.
In a normal use case, you have that form using the aggregate.list output as the source of that same form.
For the dropdown variable, not using a local variable, in the preparation assign the value you want to the corresponding variable in aggregate.list output, not to the myform.record