EndTime : {{#=}}({{issue.customfield_10590.convertToTimeZone('UTC').toMillis}} +(({{issue.customfield_10592}} + 1) * 3600 * 1000)).toDate{{/}}
issue.customfield_10590 : is the start time in datetime format
issue.customfield_10592 : is the hours in string/number format that I want to pass as an end time in a post web request call for a third-party integration.
right now when I am passing Endtime as a custom variable to the main custom body I am getting this error
Your outgoing web request configuration is invalid. Please fix the following errors and try again. Invalid smart value expression in custom body, please see smart values in our documentation for help with syntax.
please help me on this one.

Full Custom Body
{ "data": { "type": "downtime", "attributes": { "scope": "*", "monitor_identifier": { "monitor_tags": ["******"] }, "display_timezone": "Asia/Kolkata", "message": "Message about the downtime", "mute_first_recovery_notification": false, "notify_end_states": [ "alert", "warn" ], "notify_end_types": [ "canceled", "expired" ], "schedule": { "end": "{{EndTime}}", "start": "{{issue.customfield_10590.convertToTimeZone('UTC')}}" } } } }