I understand you want to assign Microsoft.Authorization/roleAssignments/write permissions to user to run application.
Microsoft.Authorization/roleAssignments/write is one of action in Role-Based Access Control (RBAC). To grant this permission to a user account, you need to assign the appropriate RBAC roles.
You can assign Role-Based Access Control administrator or User Access Administrator to your account. Along with Microsoft.Authorization/roleAssignments/write permissions, these built-in roles include several other permissions. To see the complete list of actions available in these roles, please refer to this documentation.
To assign roles you require at least you require Role Based Access Control Administrator or User Access Administrator and follow below steps
1.Go to Azure Portal → Subscription / Resource Group / Resource (where the permission is needed).
2.Navigate to Access Control (IAM) → Role Assignments.
3.Click + Add Role Assignment.
4.Select User Access Administrator or Owner.
5Assign it to the Service Principal, Managed Identity, or User, Click Save.
If you do not want to grant additional permissions included in built-in roles, you can create a custom RBAC role with only the Microsoft.Authorization/roleAssignments/write permission and assign it to the user.
For steps on how to create a custom RBAC role, please refer to this document
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