Put something like the following in the initialization handler for the manager field
var requestor = api.getRequester();
var reqfn = requestor.data.FirstName;
var reqln = requestor.data.LastName;
console.log('Requestor data is ' + requestor);
console.log('Requestor first name is ' + reqfn);
console.log('Requestor last name is ' + reqln);
Then F12 on your browser to see the logs. This will show you what data is available
Presumably you want something like
var requid = requestor.data.UserId;
(check the exact syntax).
Then in order to write that value into the attribute, you use the command that Gil showed you
prop.value = requid;
Pearse