You can add user-assigned managed identity to the resource in another subscription by adding the following to your ARM template.
"identity": {
"type": "UserAssigned",
"userAssignedIdentities": {
"/subscriptions/<Azure-subscription-ID>/resourceGroups/<Azure-resource-group-name>/providers/Microsoft.ManagedIdentity/userAssignedIdentities/<user-assigned-identity-name>": {}
}
Here's a sample ARM template that does the same for Azure Logic Apps, instead of storage for your reference: https://learn.microsoft.com/en-us/azure/logic-apps/create-managed-service-identity?tabs=consumption#create-user-assigned-identity-in-an-arm-template
If you have any questions, let me know in the "comments".
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