You are not able to delegate onboarding tasks (Write welcome message, assign a buddy etc..) to different manager.
SAP SuccessFactors Onboarding
Currently, only the checklist items task can be delegated. Delegate other Onboarding tasks is not supported at present.
This is a feature limitation and we always encourage ours customer to provide the feedback and submit a enhancement request to have this new functionality within the future releases. For detailed steps on how to raise an enhancement request please follow the Knowledge Base Article 2090228 - How to Submit Ideas for SAP SuccessFactors Products
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