Use the following jQuery code to populate a people picker field on a SharePoint form using jQuery and the CurrentContext object within a browser session

var loginName = _spPageContextInfo.userLoginName; //get login name form current session in browser
var pplPckr = $("span[data-displayname='Requested by'] .sp-peoplepicker-editorInput")[0]; // adapt this to any query selector to point to the input field with the class "sp-peoplepicker-editorInput"
var peoplepicker = SPClientPeoplePicker.PickerObjectFromSubElement(pplPckr) // Use SPClientPeoplePicker to get the actual picker object
peoplepicker.AddUserKeys(loginName); // Add the user login name to the key table to resolve the user