Using String library keywords you can fetch respective content from location. Following is the sample snippet for you
*** Settings ***
Library SeleniumLibrary
Library String
*** Variables ***
*** Test Cases ***
Get Login Challenge
Open Browser ${my_website} chrome
# assumes page redirected to your custom page i.e, <domain>/uas/portal/auth/login?login_challenge=c29d71cc5e704eb28973fab436c4b1ac
${redirected_url}= Get Location
# get login_challenge from ${redirected_url} using String library keyword
${login_challenge}= Fetch From Right ${redirected_url} ?login_challenge=
Log To Console \n${login_challenge}