For adding a custom drop-down field in task window in Outlook, please do as follows:

1. In the Tasks view, please click Home > New Task to create a new task.

2. In the opening Task window, please click Developer > Design This Form. (Note: Click to know how to add developer tab on Ribbon in Outlook.)

3. Go ahead to click the (P.2) tab, and then click Developer > Control Toolbox. See screenshot:

4. Now the Toolbox is opening. Please drag the ComboBox and drop it under the (P.2) tab. See screenshot:

5. Right click the inserted ComboBox, and select Properties from the context menu. See screenshot:

6. Now the Properties dialog box comes out. Please go to the Value tab, and click the New button. See screenshot:

7. In the opening New Column dialog box, name the new column, specify Text from both Type and Format drop down lists, and click the OK button. See screenshot:

8. Now you return to the Properties dialog box, please type the drop down values in the Possible values box, and click the OK button.
Note: Please separate the drop down values with semicolon ;, such as A;AA;AAA;AAAA;AAAAA. See screenshot:

9. Click Developer > Run this Form to show the task window in normal view, next click Task > P.2, and then you will get the custom drop-down field. See screenshot:

Note: If you need to save the custom drop-down field for reusing in future, (1) in the Untitled – Task (Design) window click Developer > Publish > Publish Form; (2) in the Publish Form As dialog box, specify the destination folder, name the form, and click the Publish button as below screenshot shown:

For reusing the custom drop-down field, please go to the main interface of Outlook, click Developer > Choose Form to open the Choose Form dialog box, and then select and open the form with custom drop-down field.

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