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I need to populate one field when I click on reject.

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&Comments=Hello

Example: https://your_sf_instance/p/process/ProcessInstanceWorkitemWizardStageManager?id=04iN0000000VCmL&Comments=Hello

However You won't be able to override the Approve/Reject actions directly.

But you can create a custom button (e.g.: VF page) on your parent object, retrieve the id of ProcessInstanceWorkItem (04iN0000000VCmL) in apex and redirect the user to the link above including your comment.

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One can do this as part of the Approval Process, in fact the wizard helps guide one through the entire process.

Below is an example where one can update the "description" field if the record is rejected during the approval process.

Is that enough info for you to go on?

Approval Process