def format_date_for_api_request(date_str):
    """
    Formats a date string (YYYY-MM-DD) into 'MM%2FDD%2FYYY' format.

    >>> format_date_for_api_request('2017-10-17')
    '10%2F17%2F2017'
    """
    year, month, day = date_str.split('-')
    return '{}%2F{}%2F{}'.format(month, day, year)