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To have the assigning process of Approvers field automated you could follow the follow these steps:
Create a custom field that fetches the manager of the reporter
Start by creating a custom field with Manager as a Microsoft user attribute and Reporter as a reference user field . Follow https://communardo-products.atlassian.net/l/cp/6z0k0ZUf for more details on how to create the desired custom field.
Define the trigger condition
The trigger condition needed to be defined for this automation rule is “Field value changed”. The automation rule should monitor for changes in the created custom field.
Define the condition
If the case is that you need the auto-assigning process to take place directly after a Jira issue is created you can use conditions that would limit the automation rule to run only on the created Jira issues within a certain period of time, for example: less than one minute ago.
Define the actions to be taken
Once the trigger condition is met, the automation rule should take the appropriate actions. In this case, the automation rule should set the approvers value to be the manager of the reporter. For this is used smart value {{custom_field_name}} or {{custom_field_id}}.
The article outlines how to automate approver assignment in Jira Cloud using custom fields and automation rules. It includes instructions for setting up the rule, but notes that auto-assignment during issue creation cannot be achieved using the "Issue created" trigger.