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You can had (or replace ) this section into the file:  ...\UIModule\service_order_maint\service_order_maint.uim (on the filestoragepath folder.)

This is an example : refno (Référence) is on the second position (=sequence section ) (for information the first position start at 0)

and the label is “Référence”.in FRA lang_name...

  <TopTitle>
    <file_name>form_asp/service_request_maint.shtml</file_name>
    <value>refno</value>
    <sequence>1</sequence>
  </TopTitle>
  <TopTitleLabel>
    <text>Référence</text>
    <lang_name>FRA</lang_name>
    <foreign_key>form_asp/service_request_maint.shtml</foreign_key>
    <foreign_key2>1</foreign_key2>
  </TopTitleLabel>  

You can control the value ...

The value in the field is “123”

I think you mean how to use Customizer to add this value, right?  

The solution above is manually making a change to the UIM that is normally done by Customizer.  I am sure it works but feel it is a bit risky adding manually and you have to track very carefully other items in the sequence so as not to create duplicates, etc.

You could also try using the title editor in Customizer to add the refno field and let that worry about the positioning, etc.

Using this method, I was able to add the referene number to the title on the service order:

I would agree with Phil that manual changes are risky. Also, you may need to check and verify your changes after every upgrade as they can be overwritten by the installer.