Setting Regarding Field on PhoneCall Activity Form

Hi,

I am looking for some code in JS to set regarding field on PhoneCall Activity through Parameters. Following is the code that I am using.
I know MS says it is not possible. Open for unsupported changes as well. Can someone help.

function buttonClicked() {
var url = "https://******.crm.dynamics.com/main.aspx?etn=phonecall&pagetype=entityrecord&extraqs=";
var extra = "";
var hist = "";

extra += "&phonenumber=XXXX567890"
extra += "&subject=Issue in Recently Purchased Item";

/*
extra += "&regardingobjectid={b475B158C-541C-E511-80D3-3863BB347BA8}";
extra += "&regardingobjectidname=A. Datum";

extra += "&regardingobjectidtype=account";

extra += "&pType=1";
extra += "&pID={b475B158C-541C-E511-80D3-3863BB347BA8}";
extra += "&pName=";

extra += "&partytype=1";
extra += "&partyid={b475B158C-541C-E511-80D3-3863BB347BA8}";
extra += "&partyname=";
*/

hist = "&histKey=647827476&newWindow=true";

window.open(url+encodeURIComponent(extra)+hist, "", "status=0,resizable=1,width=1000px,height=600px");
}