Auto Fill Text

I have a query based drop down to select certain users who meet certain criteria.
From that I wish to populate another field with a certain value from their comprofiler table.
So for example I populate the dropdown with "userx" then I would like another field to be populated with "userx_member_number"

Is this possible please and how?

Sounds like you could just use substitutions in a new text field.
[fieldname of your userx query field][user_id] might do the trick. Otherwise see www.joomlapolis.com/support/tutorials/107-use-cases/18353-using-substitutions-throughout-cb

Hi, sorry my explanation was not full enough, the substitution works fine, however what I really need to do is then store that value in the table of the user.

Hi, Thanks again for your help, I think I didn´t explain too well what I am trying to achieve.
So the user "userx" has to be assigned to a teacher, which is easily done using a query dropdown.
The teacher also has a "teacher number" which is independent of the joomla id and I also need to display and store this on the "userx" comprofiler table, so when I select the teacher it will automatically do this.

Thanks in advance for any help you can give

The only reason for it to be stored is if you need to search on it. If you don't need to search on it you can use a CB Query Field to pull its display into their profile.

If you need it searchable then create the query field to pull its display then have a text field that's set to read only. Next use CB Auto Actions on the after profile update and after registration triggers (frontend and backend) to update your text field with the value of the query field by substituting in the query field value.

Alternatively query action would also work to pull the value and update their row with it. Just depends on which you're more comfortable with, both will require writing some SQL.