Hello Ryota
You need to set the number range to allow the special characters.
The Service Agent ID should be entered based on the range maintained in the fine tuning activity Number Range for Service.
If you would like to have a number range which supports all combination of alphabetics, numeric, alphanumeric and special characters (_-/:\.) but with interval type as Manual, you can use the below number range for Service.
Start Number: 0
End Number: Z9999999999999999999Z
Interval Type: Manual
Note: End Number should start and end with 'Z' and you can define number of 9's based on the length of the Service.
Alternatively,you could also refer to the case document 1702694 - Error Message: Service ID is not valid; While Creating a New Service for more details.
Thanks
Parveen Kumar