Make a field required but only in one company

The RowSelected event will work perfectly for this. You can decide how you want to determine the logic for making the field required/not required and call this. I’d be tempted to add a field to the relevant Preferences screen that toggles this behaviour on and off. That way if they add a new company, you don’t need to recompile anything - just check, or uncheck a box.
 

protected virtual void _(Events.RowSelected<SOOrder> e, PXRowSelected baseHandler)
{
  baseHandler?.Invoke(e.Cache, e.Args);

  SOOrder row = e.Row as SOOrder;
  if (row == null) return;

  bool flagVariable = MethodUsedToDetermineRequiredOrNot();

  PXUIFieldAttribute.SetRequired<SOOrder.usrYourField>(e.Cache, flagVariable);

}

I also have this method that I found within the Acumatica code that I use to check to make sure that all required fields have been filled so I can control that better:

if (CheckRequiredFieldsFilled(Base.Document.Cache, row) == false)
{
  PopupNoteManager.RegisterText(Base.Document.Cache, row, "OrderNbr", "Please enter the missing field data.");
}


private static bool CheckRequiredFieldsFilled(PXCache aCache, IBqlTable aFilter)
{
  bool allFilled = true;
  foreach (var field in aCache.BqlFields)
  {
    PXFieldState state = aCache.GetStateExt(aFilter, field.Name) as PXFieldState;
    if (state != null)
    {
      object value = aCache.GetValue(aFilter, field.Name);
      if (state.Required == true && value == null)
      {
        aCache.RaiseExceptionHandling(field.Name, aFilter, null,
          new PXSetPropertyException(Messages.RequiredField));
        allFilled = false;
      }
    }
  }
  return allFilled;
}