If you are looking for a way to grant SendAs for a cloud/hybrid user on an on-premise mailbox, you can use the following:

Note: Mailbox permissions and capabilities NOT supported in hybrid environments. 

Azure AD Connect doesn't automatically synchronize Send As permission between on-premises Exchange and Microsoft 365 or Office 365, so cross-premises Send As permissions aren't supported. 

However, Send As will work in most scenarios if you manually add the Send As permissions in both environments, using Exchange Management Shell for on-premises Exchange and Exchange Online PowerShell for Microsoft 365 or Office 365.

Run the following command in the Exchange Management Shell on your on-premises Exchange server:

Add-ADPermission -Identity EXO1 -User ONPREM1 -AccessRights ExtendedRight -ExtendedRights "Send As"

Then run the corresponding command in Exchange Online PowerShell:

Add-RecipientPermission -Identity EXO1 -Trustee ONPREM1 -AccessRights SendAs

In Coreview:

   "id": "036b8924-6800-43f1-a219-d1d963da6719",
   "title": "Exchange - Assign SendAs permission hybrid",
   "description": "Assigning SendAs permission for a cloud user on an on-premises mailbox",
   "lastModified": "2022-10-04T09:50:05.2560000Z",
   "target": "Mailbox",
   "tags": [],
   "vars": [
         "name": "TargetMailbox",
         "type": "string",
         "isRequired": true
   "params": [
         "name": "UserPrincipalName",
         "type": "string",
         "isDefault": true
   "columns": {
      "UserPrincipalName": "",
      "CustomAttribute10": "",
      "DistinguishedName": "",
      "OnPremisesDistinguishedName": "",
      "CustomAttribute13": "",
      "Alias": ""
   "version": 3,
   "statement": "param ([string]$UserPrincipalName, [string]$TargetMailbox)\r\n\r\nAdd-RecipientPermission -Identity $TargetMailbox -Trustee $UserPrincipalName -AccessRights \"SendAs\" -Confirm:$false"