you can follow these steps:

1. Go to the CRM module in the Odoo backend.
2. Click on the "Activities" menu to access the activities view.
3. Click on the "Filters" button at the top right corner of the view.
4. Select "Manage Filters" from the dropdown menu.
5. Click on the "Create" button to create a new filter.
6. In the "Name" field, enter "Today" or any other desired name for the filter.
7. In the "Model" field, select "Lead/Opportunity" or the appropriate model.
8. In the "Domain" field, enter the following domain:

​["&", ("activity_ids.date_deadline", ">=", datetime.datetime.combine(context_today(), datetime.time(0, 0, 0))),
("activity_ids.date_deadline", "<=", datetime.datetime.combine(context_today(), datetime.time(23, 59, 59)))]

1. Click on the "Save" button to save the filter.

Now, when users select the "Today" filter from the search filter dropdown in the CRM activities view, it should show activities with a deadline for the current day.

Note: Make sure to import the necessary modules at the top of your Python file, such as datetime and fields.