RE: Setting an Extender Field (Drop down list) to default to a certain option help
I believe you would need to set up a template on your Extender Form. A template is basically one way of having fields default to pre-defined values to minimize data entry for those using the form. Since I don't know exactly what is on your form, here's a high level walk through but you need to see what works for the values you have.
My client is using GP 2013 R2 so that is the version of Extender I'm referring to, in case any of what I mention is newer functionality?
In Extender Forms, click on Templates on the menu bar.
In Templates, assuming there isn't one already, click Add to create a template. Give the template a name like "Default Fields Template" or something that makes sense to you. In the Add Template window, click Add to choose which field you want in your template (your DDL), and what the default value you want. If that is the only field you want with a default value, save that template.
Back in the Templates window, at the bottom of the screen, there is a checkbox "Use template to enter default values" and then you can choose which template has the defaults you want to use for most people.
Save that, and that should do the trick.