Prefilling data on an http post request

How to configure prefill for an app when creating a new instance from a http post request.

Altinn apps support instantiation with prefill. The form data is attached in a multipart in the instantiation request that is sent to the app. Below you will se an example of a requeest to instantiate an app with prefill for partyID 12345.

Content-Type: multipart/form-data; boundary="abcdefg"
Body:

--abcdefg
Content-Type: application/json; charset=utf-8
Content-Disposition: form-data; name="instance"

{
    "instanceOwner": {
        "PartyId" : "12345"
    }
}

--abcdefg
Content-Type: application/xml
Content-Disposition: form-data; name="Endring-av-navn"

<?xml version="1.0"?>
<Skjema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" skjemanummer="1533" spesifikasjonsnummer="11172" blankettnummer="RF-1453" tittel="Endring av navn" gruppeid="9308">
<Innledning-grp-9309 gruppeid="9309">
    <NavneendringenGjelderFor-grp-9310 gruppeid="9310">
    <SubjektFornavnFolkeregistrert-datadef-34730 orid="34730">Ola Nordmann</SubjektFornavnFolkeregistrert-datadef-34730>
    </NavneendringenGjelderFor-grp-9310>
    <Kontaktinformasjon-grp-9311 gruppeid="9311">
    <MelderFultnavn orid="34735">LANGØY MADS</MelderFultnavn>
    </Kontaktinformasjon-grp-9311>
</Innledning-grp-9309>
</Skjema>

--abcdefg--