String Manipulation and TCL Error

You should be able to use this code instead. You need to parse out the UserId before you can use it. This is one way of doing that (as long as your parameter value is consistent in format). There's other methods of splitting a string to get values, but I think this will work.

set param "accountname=i%3A0%C7%B5%2Et%7Cadfs%20stage%7CIAmAUserName"
log local0. "Query: $param "

set lastpipe [expr [string last "%7C" $param] "3"]
log local0. "  Last pipe Index: $lastpipe"

set UserId [string range $query $lastpipe end]
log local0. "  Username: $UserId"

set newparam "p%5Bid%5D=i%3A0%C7%B5.t%7Cadfs%7C${UserId}&p%5BidType%5D=adfs"
log local0. "  New parameter value: $newparam"

 In 11.5+ you can do this
set newquery [string map [list $param $newparam] [HTTP::query]]
log local0. "  New Query: $newquery"
HTTP::query $newquery
 Otherwise you can do this
set newuri [string map [list $param $newparam] [HTTP::uri]]
log local0. "  New URI: $newuri"
HTTP::uri $newuri