Where to create & Add a Custom button in Page layout-> Activity history section?

Customizing Related Lists

Available in: All Editions

User Permissions Needed To customize related lists: “Customize Application”

To customize the buttons, columns displayed, column order, and record sort order of related lists on record detail pages in Salesforce and the Salesforce Customer Portal:

1. Access the page layout editor.
2. Double-click a related list on the layout to edit it. If you are using the enhanced page layout editor, you can also click the wrench icon (_img alt="Wrench icon" src="https://cs3.salesforce.com/img/ple/wrench.gif" /_).
3. To customize the fields that display in the related list:
   • Select one or more fields and use the arrows to add or remove the fields to the related list columns on the page layout, and to define the order in which the related list columns display. You can include up to 10 fields per related list.Note
     _img src="
     https://cs3.salesforce.com/img/help/helpNote_icon.gif" /_The first field of a related list can't be moved. They are considered as unique identifiers for the record.

   • Use CTRL+click to select multiple fields individually.
   • Use SHIFT+click to select multiple fields as a group.
   • Select a field from the Sort By drop-down list to sort the items in the related list, which will be displayed in ascending order unless you select Descending. The default sort order varies per record. The Sort By drop-down is not available for activities and opportunity products.
   • If necessary, select additional page layouts to apply your related list customizations to. Only layouts that include this related list appear in the list. Layouts that include related lists with the same customizations as the current layout had when you opened are selected by default.

   Lookup fields are not available for display on their corresponding lookup related list. For example, the case lookup field on an account page layout is not available when editing the cases related list.

4. To customize which standard buttons display in the related list, select or deselect the checkbox next to any standard button name.Note
   _img src="
   https://cs3.salesforce.com/img/help/helpNote_icon.gif" /_To view the buttons in the enhanced page layout editor, click the plus sign (+) in the Buttons section.

5. To customize which custom buttons display in the related list:
   • To add or remove a custom button, select the button and click Add or Remove.
   • Sort custom buttons by selecting them and clicking Up or Down.

   To create a custom button for the related list, see Defining Custom Buttons and Links. The custom button must be defined for the object contained in the related list, instead of the parent object, and the button Type must be List Button. For example, to display a custom button on the Contacts related list of an account, define the custom button for contacts, not accounts.

6. If necessary, click Revert to Defaults to undo any customizations and use the default Salesforce settings in the related list.
7. Click OK to store your customizations. Changes are not saved until you click Save on the page layout.
8. Select the Overwrite users’ personal related list customization to apply the related lists in the page layout to all users, even if they have already customized their display.
9. Click Preview to review the page layout. From the preview in Enterprise, Unlimited, and Developer Editions, select a profile to see how the pages will look for users with different profiles.
10. Click Save.

Note

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for example- I created a button called "course status" in "opportunity" object & I was able to find it in the "Account" object and added it, but it was not displayed  in Account tab when creating new account. Please clarify

You can follow these steps:

1. Go to Setup - > Activities ->Tasks button and Links ( Or Event button

)

3. Go to the Object Page Layout (Standard or Custom object) and then go to the

Activity History Related List. 

You are done.