I ran into this same issue.  What you can do is set the 'ShippedQuantity' on the order detail lines, and that will create a shipment (but not an order). 

Hi, tried the above solution but for me shipment is generating with order. I need shipment without order. Below is the Json

{"CustomerNumber":"1105","ShipmentNumber":"SHPT126","SHIDATE":"2021-02-06T11:10:32.249Z","OrderDetails":[{"Item":"A1-310/0","Location":"1","QuantityOrdered":2,"QuantityShipped":2,"PricingUnitPrice":5.0}]}

Kindly help me

Looks like I was wrong.  I ran a quick test and your right, the Order document is also created.  But it also appears to be completed - so shouldn't cause any harm.  Why do you not want to create an Order document?

Because our customer has separate customized Order Entry so they don't need sales order.