Use the following steps to go to the Presidential Election Campaign Fund selection page.

In the search box enter Presidential Election Campaign Fund and click search.

Select jump to Presidential Election Campaign Fund.

You will now have the option to contribute or not contribute to the Presidential Election Campaign Fund.

But if you mean "where do I deduct money given to a Presidential campaign", the answer is--you don't.  It may or may not be a good cause, but it isn't a charitable contribution.

In my 2021 TurboTax Home & Business Windows, under Personal taxes, Other Tax Situations, Other Return Info, there's an option for the Presidential Campaign Fund.