You can do something like this:
var req = new XMLHttpRequest();
req.open("GET", Xrm.Page.context.getClientUrl() + "/api/data/v8.0/pws_streedandhousenodatas?$filter=_pws_streetandhousenumberid_value eq " + Xrm.Page.data.entity.getId(), true);
req.setRequestHeader("OData-MaxVersion", "4.0");
req.setRequestHeader("OData-Version", "4.0");
req.setRequestHeader("Accept", "application/json");
req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
req.setRequestHeader("Prefer", "odata.include-annotations=\"*\"");
req.onreadystatechange = function() {
if (this.readyState === 4) {
req.onreadystatechange = null;
if (this.status === 200) {
var results = JSON.parse(this.response);
for (var i = 0; i < results.value.length; i++) {
var pws_streedandhousenodataid = results.value[i]["pws_streedandhousenodataid"];
}
} else {
Xrm.Utility.alertDialog(this.statusText);
}
}
};
req.send();
In this case the Xrm.Page.data.entity.getId() get you your current record id and you are looking all the lookups (that are in the sub-grid), you can also add some fields to select more info from them.