Hi @nagendrapaladi 

I have not yet tried OOTB solution , but ideally you should raise hi Case and make OOTB work, in case you don't have any other options -

1. You need to create a catalog on portal - Shop from XYZ.

2. CatLog client script which call Ajax and get all content expected by punchout such as groups, user id details and headers required to trigger punchout, you will get this information from Punchout system

3. Write redirect code, which routes to Punchout in same catalog script. 

If I find time, I will try OOTB solution.

Thanks,

Pramod