Create Drop Shipments from Sales Orders against vendor selected for the lookup custom field
Settings > Preferences > Sales Orders > Field Customization > New Custom Field
Create a Loop-up data type custom field with "Vendor" module as related entity
Settings > Automations > Custom Functions > New Custom Function
Module: Sales Order
Copy and paste the Dropshipment for SO script.
Create connection for Zoho Inventory - zom
Settings > Automations > Workflow Rules > New Workflow Rule
Module : Sales Order
Workflow Type : Event Based
When an Sales Order is: Created / Edited
Filter the triggers: When vendor lookup field is not empty
Choose the appropriate custom function in the Action.
Save the workflow. On each salesorder creation / edit action, the workflow automatically creates a Drop Shipment Purchase Order