Jira Application is running on cloud, so script runner code is in need for below scenario,
I have a custom field called approver, when ever the approver is selected while creating an issues ,that approver must be automatically assigned to that issues .
I have created an example script located here which can be placed on the last post function of the Create transition for a workflow and shows how you can extract the value of a user picker field and set the assignee as the extracted user. Using this script you just need to set the UserPickerFieldName variable to the name of your user picker field.
You will be able to modify this script to achieve your requirements if you are using a different field type for the Approvers field by referring to the examples for extracting custom field values in the documentation pages located here.
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