Hi Shifali,
sorry for the confusion,
As per to generate new alpha numeric number automatically,an assignment has to be created. I have created an assignment as per your scenario with help of Koride Sharma which may resolve your issue.
Just note if the autoid is 3 digits (i.e. 100 to 999) you can use the value as 3, but increase to 4 once the autoid is 4 digits (i.e. 1000 to 9999).
you can execute the assignment before trigerring the syndication.
Assignment :
workflow:
Output:
Thank you
Rahul