How to set display fields on form to Table field..

Hello,

I have created one Table that get lookup from UserInfoTable. Corresponding  i have create form for the same. When i click on Userid on the form  then lookup appear and i select any user name corresponding networkAlias are fill into Alias field in form. This networkAlias come from UserInfoTable of that corresponding table. Now i want to save this networkAlias to my Table field called Alias. To getting networkAlias on the form I have do following code for the same:

display SystemAccountName osAccName(II_FSA_Users  _II_FA_Users)
{
    UserInfo    userInfo;

    select firstonly userInfo where userInfo.id == _II_FA_Users.II_UserName;
    if (userInfo)
        return userInfo.networkAlias;
    return '';
          
}

I have write display method on  Form DataSource. Now how can i insert it to corresponding field Alias into  My Custom Table. Please help on this. Probably i have spent  half day on the same. It's driving me crazy.