main() Function

Accessing the active workbook

All your office scripts must have the following function declaration

function main(workbook: ExcelScript.Workbook) { 

} 

Getting the user to input a message

Add a variable to the main function and then use this variable in your script.
This allows you to display a userform asking the user for some input.

function main(workbook: ExcelScript.Workbook, message: string) { 
   let selectedSheet = workbook.getActiveWorksheet(); 
   selectedSheet.getRange("A1").setValue(message); 
} 

When this scripts runs it will prompt the user asking for a message.