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Go to the Manage Contact Forms Page. You can get there by:
Clicking My Apps in the top menu My Apps sub menu
Search Contact Forms and click it.
Search for the Contact Form you want to add the logic to in the list of forms.
Click on the name of the form to go to the Edit Page of that form.
Go to the question you want to apply the Hide Unless Logic.
Click on the Actions button corresponding to that question.
Now select Skip Logic: Hide Unless option from the dropdown.
The Hide Unless popup for that question appears.
Select the previous question which affects this question. (You can only select a multiple choice question - which can have one or limited or unlimited options Change the number of Allowed Answer Choices for a Category field in a Contact Form - that comes before this question)
This open the opens for this questions.
Now select the options for which this question is valid. (For which options the question should be visible)
Click on Save.
You can now see the Logic attached to the question
This question will now only be available/visible to answer if the selected options are selected by the user.
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You can use this form on a Signup page as well and the Hide Unless Logic will work the same.