In order to create a Custom Field, you need to create a Custom Web Form.
Manage Forms > Add Custom Web Form
Custom field
A Custom Field allows you to ask specific questions (in your form) to your subscribers in order to gather more detailed information that is not covered/asked in the default fields listed in the Form Template.
Step 1.
Create a Custom Web Form and fill out the required fields.
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Step 2.
Select the "Add a Custom Field" option : A custom field allows you to find out specific information about your subscribers that aren’t covered in the default fields listed above.
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Step 3.
Title the "Custom Field" and select if you would like to make the field required.

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Step 4.
Is this an open ended question? : Use an open ended question to allow people to write whatever they wish.
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Step 5.
Add answers : Use this if you said No to the question above.
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Step 6.
Can a subscriber select more than one at the same time?: Depending upon what you want to capture you can allow users to choose more than one answer or simply make it a single choice. Either way, the information will be captured for targeting.

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Step 7.
Save the Custom Form.