Auto Fill Date Field Based Off of Another Field

You will need to set a "dynamic form rule" to populate that date, when your status field is changed.  

If you are an Admin on the account and you open up one of the records
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In the top right you will see "Customize This Form"
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After opening up those settings, click on the "Dynamic Form Rules" tab
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Create a new rule with the needed logic
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Something to the affect of:

Condition
If [Status] is equal to "Complete"

Action:
Change [Completion Date] to the value in the field [Scheduled Date]