You can create a microflow that takes the ticket date value and uses date manipulation to get the start date. Date functions can be found here: https://docs.mendix.com/refguide7/add-date-function-calls and here https://docs.mendix.com/refguide7/trim-to-date#1-introduction
Once you create the microflow, call it as on onChange event handler on the ticket date field.
Hope that helps,
If OrderDate is always the current date you could create a variable StartDate and fill it with addMonths([%BeginOfCurrentMonth%], 2)