How to Set the Option 'Assign using active assignment rules' to True, Only When a Case or Lead is Created in Salesforce

Overview of the Solution

The configuration involves four steps:

1. Create a new page layout where the "Assign using active assignment rules" checkbox is visible and selected by default (used only at creation time).
2. Create a new record type associated with that page layout — used temporarily during record creation.
3. Create a Flow (Lightning) or Workflow Rule (Classic) that triggers on record creation and updates the record type back to the standard record type, where the checkbox is hidden or disabled.
4. Test and confirm.

Step 1: Create a New Page Layout

Classic

1. Click Setup.
2. Enter "Leads" or "Cases" in the Quick Find box.
3. Select Page Layouts.
4. Click New. At the "Create New Page Layout" screen, select your existing page layout and give it a name that indicates it is used for assignment rule creation (for example, "Case Creation - Run Assignment Rules").
5. Click the Layout Properties button on the palette. Disable the "Show on edit page" and "Select by default" Case Assignment Checkbox options and click OK. Click Save.

Lightning Experience

1. Click the gear icon and click Setup.
2. Click Object Manager.
3. Select Case or Leads.
4. Select Page Layouts.
5. Click New page layout. At the "Create New Page Layout" screen, select your existing page layout and name it for assignment rule creation.
6. Click the Layout Properties button on the palette. Disable the "Show on edit page" and "Select by default" Case Assignment Checkbox options and click OK. Click Save.

Step 2: Create a New Record Type

Classic

1. Click Setup.
2. Enter "Leads" or "Cases" in the Quick Find box.
3. Select Record types.
4. Click New.
5. At the "Create New Record Type" screen, select your existing record type. Name it to indicate it is used for assignment rule creation (for example, "Case Creation - RUN ASSIGNMENT RULES").
6. Select the Support Process or Lead Process, set it to active, and do not assign this record type to any Profiles. Click Next.
7. At the "Assign page layouts" screen, apply the page layout created in Step 1 to all Profiles. Click Save.

Lightning Experience

1. Click the gear icon and click Setup.
2. Click Object Manager.
3. Select Case or Leads.
4. Select Record types.
5. Click New.
6. At the "Create New Record Type" screen, select your existing record type and name it for assignment rule creation.
7. Select the Support Process or Lead Process, set it to active, and do not assign this record type to any Profiles. Click Next.
8. At the "Assign page layouts" screen, apply the page layout created in Step 1 to all Profiles. Click Save.

Step 3: Create a Record-Triggered Flow (Lightning Experience)

Flow Setup

1. Navigate to Setup and search for Flows.
2. Click New Flow.
3. Select Record-Triggered Flow and click Create.
4. Configure the Flow:
    1. Object: Select Case (or Lead).
    2. Trigger the Flow When: A record is created.
    3. Condition Requirements: Custom Condition Logic Is Met.
    4. Condition: The RecordTypeId field equals the ID of the "Case Creation - RUN ASSIGNMENT RULES" record type created in Step 2.
5. Click Done.

Add the Update Element

1. Click the + icon on the path and select Update Triggering Record.
2. Label: "Update Record Type to Standard".
3. How to Find Records to Update: Use the Case record that triggered the flow.
4. Set Field Values:
    1. Field: RecordTypeId
    2. Value: The 18-digit ID of your standard Record Type (the one you want the record to end up with, associated with the page layout from Step 1 where the assignment checkbox is hidden).
    3. Tip: You can use a Get Records element to dynamically retrieve the standard Record Type ID, or hard-code the 18-digit ID directly.
5. Click Done.

Save and Activate the Flow

1. Click Save.
    1. Flow Label: "Case - Run Assignment Rules Only on Create"
    2. API Name: "Case_Run_Assignment_Rules_Only_on_Create"
2. Click Activate.

Step 4: Test and Confirm

Create a new case or lead using the "Case Creation - RUN ASSIGNMENT RULES" record type. Verify that the "Assign using active assignment rules" checkbox is set to true at creation. Save the record. Then click Edit on the saved record and confirm that the "Assign using active assignment rules" checkbox is no longer visible or is set to false, and that editing the record does not trigger the assignment rule again.