How to ask for an "Holder of key" assertion ?

I have a SP which neesd a SAML assertion for himself, this assertion must contains a subject confirmationMethod "Holder of Key", and the associated keyInfo to identify its own key or certificate.

What is the good way to ask an IDP for such an assertion ? Can I ask explicitly for an "Holder of Key" ? Where do I put the keyInfo in the request ?

Thanks in advance...

Val廨ie BAUCHE