I had a task today to open new task form with Regarding field prefilled. Based on SDK description I tried to use following code to accomplish my task:
varparams = {
regardingobjectid: referenced record id,
regardingobjectidtype: referenced record type name,
regardingobjectidname: referenced record display name
};
Xrm.Utility.openEntityForm("task", null, params);
I knew that this feature works for out of box buttons so I rechecked URL for activities that are created with OOB buttons and reworked code to following which worked without issues:
varparams = {
pId: referenced record id,
pType: referenced record type code,
pName: referenced record display name
};
Xrm.Utility.openEntityForm("task", null, params);