Visual Studio Workflow Set People Field

I am new to Visual Studio Workflows, so please excuse my ignorance. I am having an issue setting the value of a People field. I have an Account Id coming from an InfoPath form. I need my workflow to take that value and insert/convert it to the value of the Person column.

Account Id

domain\username

Workflow Code

private void onWorkflowActivated(object sender, ExternalDataEventArgs e)
{            
    String acctId = (String)workflowProperties.Item["Account Id"];
    acctId = "-1;#" + acctId;//I've been told this is the format the Person column is expecting
    workflowProperties.Item["Stakeholders"] = acctId;

    var s = workflowProperties.Item["Stakeholders"];//This is just a test
}

The current result is that the Stakeholders field is null. I would like it to be a Person Object with the value provided by the Account Id column. Any help would be much appreciated.