COMPAIGN:
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Step 1: To open the App Launcher, click Campaigns
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Step 2: Select a campaign for which you want to set as the default and check the is default box,then save it.
LEAD/CONTACT:
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Step 1: From Setup, enter Object Manager and select contact or lead
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Step 2: Choose the Fields & Relationships and click Lead Source.
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Step 3: Find the Account/Lead Source Picklist Values ,select a Values for which you want to set as the default.
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Step 4: Find the check box after the default, check it, check and save it