Hi,
you can use these settings:

Table: #__users
Column for value: id
Column for text: name

in table parameter you can use #__users or lejxa_users. "#__" is a Joomla constant and will replace with table prefix.
For parameter "Column for value" we chose id, this because is the only value associated with the user that never changes.

Don´t work

can you put one example in my site pleasse?

hi, see now your "Cliente" field. you have not filled column for value and column for text parameters.