There are 2 ways to trigger a route manually 

1. From Replenishment view by selecting product, To Order Qty, Route and ordering

2. With Push rules, which trigger subsequent operation when products arrive at a specified location

Since your intention is to have an separate operation type. Here are the steps to achieve it with push rules. (The assumption here is that the 3-step shipping route is default Odoo route.)

1. Create a new operation type as internal transfer with source location stock and destination as Packing (Inventory > Configuration > Operations types)

2. Create a new route with rule (Inventory > Configuration > Routes)

Action - Push to 

Operation type - newly created operation type

Source location - Stock

Destination location - Packing zone

3. Additionally in the receiving WH (BUR) a create another Push rule from Transfer to Input (Inventory > Configuration > Rules)

To make a transfer, create a new operation add products with customer location as your company WH and validate (subsequent operations are created on validation and by marking as 'To Do')

Consistent with 3-step delivery, once the 1st operation is validated, Pack operation is triggered followed by Delivery. As the customer is your WH, the delivery location would default to Transit.

On validating delivery order, a receipt would created in the receiving WH

You could also achieve the same, by directly creating a Pick operation in the source location.

Do ensure the WH is created as a contact and has the customer location configured to Inter-warehouse transit location. This field is visible on the contact > Sales & Purchase tab > Inventory section. 

Hope this helps.

For a manual transfer between warehouses, you typically do not need a custom Operation Type that manually chains all the pickings. Instead, configure an Inter-Warehouse Transfer Route that uses a Transit Location.

Your route should define pull rules similar to:

• Transit → BUR/Input

• BUR/Input → BUR/Stock

• SOF/Output → Transit

When you create a transfer from SOF to BUR (or a replenishment generated through the route), Odoo will create the required chained pickings automatically according to the warehouse reception and delivery steps.

A common reason why no transfer is generated is that the route is not assigned to:

• The product, or

• The product category, or

• The warehouse replenishment configuration.

Also verify that:

• Multi-Step Routes are enabled.

• Multi-Warehouses are enabled.

• The Transit Location is configured as an internal/transit location.

• The route rules have the correct source and destination locations.

• The procurement group is created from a demand (replenishment, reordering rule, or transfer request).

If your goal is to manually select products and create a transfer request, Odoo's standard Replenishment Between Warehouses feature is usually the recommended approach. With the route configured correctly, Odoo will generate the necessary pickings across both warehouses, including the transit movements, without creating each operation manually.