Below is the workaround:-
use the payload below in the API, and it worked..
<MaintenanceSchedule>
<account_id>-1</account_id>
<description>API Test Schedule</description>
<name>API_Test_Schedule</name>
<start_date_time>
<day>09</day>
<month>07</month>
<year>2026</year>
<timestamp>
<hours>14</hours>
<minutes>10</minutes>
<seconds>00</seconds>
</timestamp>
</start_date_time>
<end_time>
<type>duration</type>
<duration>
<hours>2</hours>
<minutes>0</minutes>
<seconds>0</seconds>
</duration>
</end_time>
<timezone>Europe/Berlin</timezone>
</MaintenanceSchedule>