Hey Christina,
You can still leverage drop-down values when selecting "default value" fo ra hidden field.
In order to do so, add a field to your form and select "Field Type" = "Select":
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From here, click "Edit" next to "Values" to define the values you want in this picklist. You can use either the simple or advanced editor here, depending on your preference (shown: simple)
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Now change the "Field Type" to "Hidden":
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when select "Edit" next to autofill, your default values now show up as a drop-down menu from which you can select a value:
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To re-use this for a future form, you can clone the form and this will be carried over to the new asset.
If you have any questions, please let me know.
Thanks,
Jason