abc="Create [newSystem=System [identityDomain=bbundlesystemser201201-test, admin=AdminUser [firstName=BSystemAdminGivenName, middleName=null, lastName=BSystemAdminFalilyName, userName=bbundlesystemadminusername, password=*******, email=hello@example.com], idmConsoleURL=https://abc.com.jspx, sftpHost=d910.abc.com, sftpUser=3pyylzoo, sftpPwd=*******]]"
echo $abc
sed -n 's/^userName= //p' "$abc"

userName=bbundlesystemadminusername

$ declare -A a
$ while IFS== read k v; do a["$k"]="$v"; done < <(grep -oEi '[a-z]+=[^], ]+' <<<"$abc")
$ printf '%qn' "${a[userName]}"
bbundlesystemadminusername

[[ $abc =~ userName=([^][,[:space:]]+) ]] && userName="${BASH_REMATCH[1]}"
echo "$userName"

bbundlesystemadminusername

userName=$(sed 's/.*userName=([^][,[:space:]]*).*/1/' <<< "$abc")