cartpauj messages TO field cannot find valid user in Participants DB

Subject: How to link to members of participant database to send messages using cartpauj.
No matter what is put into the ‘TO’ field in messages, I get “no matches found” and ”you must enter a valid recipient”.
Secondly, how is sender identified?

What I have done:
The member profile page, [pdb_single], has following link to send private message:
/messages/?pmaction=newmessage&to=[content field=”username” meta=”ID”]”>Send a message
according to suggestion by karel772 at
https://www.ads-software.com/support/topic/autofill-feild-using-url for obtaining the username and ID of database member.

Page with slug messages containing [cartpauj-pm]
Downloaded plugin ‘custom content shortcode’.

Any help would be appreciated.
Thanks, Morgan