Navigate to the Configure page by clicking on the settings icon in the upper right-hand corner
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Click on Custom Options from the menu, then click Candidates
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Click Add field
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Fill out the details of your new custom field:
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It should look something like this:

After saving, scroll down to the bottom and store the Field key – we'll need this value to map the country codes
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Next, find a job that you want to add the custom application question to (we'll be applying the new question to all applicable job posts later)
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Click on Job Posts from the menu on the left then click on the pencil icon to edit the post
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Scroll down to the Custom application questions section and click Add custom question
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Fill out the details of the new custom question:
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Click Save on the job post edits
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Now, head over to the Jobs tab > select all jobs then click on Bulk actions
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Select the Custom Options tab at the top
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Find the Country Prefix question we created earlier and click on the 3 horizontal dots on the far right. Select Make Required for All Job Posts.
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Click Save and Exit at the bottom. Greenhouse will confirm the changes with this Saving in Progress popup
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You're all set! You'll see your new question on your external job posts like in this example:
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