Add Custom Activity Field to Task Page Layout In Salesforce Classic

NOTE
You’ll only have to follow these steps if you didn’t select your desired page layout in Step 9 above.
  1. In Salesforce, click Setup.

  2. Type “Task” in the Quick Find box.

  3. Click Task Page Layouts.

  4. Click Edit next to the task page layout you’d like to add this field to.

  5. Drag and drop the field to the desired section of the Task page layout.

  6. Click Save.

Salesforce Lightning

Create Custom Activity Field in Salesforce Lightning

  1. In Salesforce, click on the gear icon on the top right and select Setup.

  2. Click Object Manager.

  3. Type “Activity” in the Quick Find box.

  4. Click the Activity label.

  5. Click Fields & Relationships.

  6. Click New.

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