En la propiedad Update del campo (no del control) poner esto:
{'@odata.type':"#Microsoft.Azure.Connectors.SharePoint.SPListExpandedReference";
Id:ddlSubArea.Selected.ID;
Value:ddlSubArea.Selected.Organization}
https://powerusers.microsoft.com/t5/Building-Power-Apps/LookUp-Field-not-updating-i-n-sharepoint-list/m-p/375831#