You can use Outlook's built-in Advanced Find feature:
1. Select the mail folder, then click on the Search input box, and select Search Tools
> Advanced Find
2. In the pop-up dialog, go to the Query Builder
tab, then click Field
> User-defined fields in folder
, and select the filed your custom field.
3. At last, set Condition and Value, and then Add to List, and click Find Now.