How can we add calculated custom fields on a process in Azure Devops Boards

Azure DevOps support rollups in the backlogs only: Display rollup progress or totals. To use your own logic, you can:

1. Create an extension (for example WSJF (Weighted Shortest Job First)).
2. Use az boards work-item to get, calculate, and update fields.
3. Use Excel to create calculated columns and publish fields: Bulk add or modify work items with Excel

If you mean creating a custom field Calculated to your project process. You can follow below steps.

1, Go to organization settings -->Process-->Select the process which your project is currently using.

If your project is using the System processes: Agile, Basic, Scrum, and CMMI. You need to create Inherited processes. For System processes are locked from being changed. See System versus inherited processes.

After you select your project process. You can select work item types to add the custom field. See Add a custom field.

2, If your project was using System processes. After you created the Inherited process, and added the custom field. You need to change your project process to this newly created inherited process. See Change the project process.