Mutual SSL Authentication for inbound API testing with Self Signed Certificate

[GeoV]

I want to test for Mutual SSL authentication to a PID instance. I do not have Client SSL from a CA Root. So it is possible to test with a self signed certificate

OR 

Is there a functionality  in ServiceNow to download a private key and certificate - just for testing with postman

[GeoV]

Many  thanks , Really good concise feedback.

One last question - My third part vendor will also client id and client secret. In addition to that, to add extra security, they can they can also Client SSL validation. So my question -  in addition to oAuth API authentication  - can we also do Client SSL Certificate validation.

My understanding, if the client SSL validation fails which happens ADCv2, then the API call will not  proceed  to Authenticate via OAuth 2 . If  both (OAuth and Client SSL validation) are possible , how to setup access to  insert new records via a table api - which is a JSON document