How to create an Obj inside a particular workflow

Depends a bit on how and where you create the object. From the way you ask it seems like you attempt to do it via scripting or something of that sort. If that is the case you need to get the workflow ID and insert it as a lookup into property 38. You should also get the first workflow state ID and insert that as a lookup into property 39.

Here is a script that you can use as inspiration:

'General script for changing workflow and or state as State Action
'2021.07.09 Karl Lausten
Option Explicit
'clear State
Dim oStProperty : set oStProperty = CreateObject("MFilesAPI.PropertyValue")
oStProperty.PropertyDef = 39
oStProperty.TypedValue.SetValue MFDataTypeLookup, Null
Vault.ObjectPropertyOperations.SetProperty ObjVer, oStProperty
'prepare WF
Dim oWFProperty : set oWFProperty = CreateObject("MFilesAPI.PropertyValue")
oWFProperty.PropertyDef = 38
'determine future workflow and state (if conditions are required, insert relevant "if" statements)
Dim iWFnew : iWFnew = Vault.WorkflowOperations.GetWorkflowIDbyAlias("<your workflow alias>")
Dim iWFnewState : iWFnewState = Vault.WorkflowOperations.GetWorkflowStateIDbyAlias("<your state alias>")
'set new workflow and state
oWFProperty.TypedValue.SetValue MFDataTypeLookup, iWFnew
Vault.ObjectPropertyOperations.SetProperty ObjVer, oWFProperty
oStProperty.TypedValue.SetValue MFDataTypeLookup, iWFnewState
Vault.ObjectPropertyOperations.SetProperty ObjVer, oStProperty

Thank you very much!! It worked!