The code below is my solution in creating a custom SharePoint List form. The list has a lookup column, the column is filled with Name (Fullnames) of employees. The task is to mark option with user Name as selected. Variable 'user' is equal to Name of employee.

The Javascript with jQuery code below:

$("select[title='Coach'] option").each(function() {
        if($(this).text() == user) {
                $(this).prop('selected', true);
        }
 });