Okay, time for the solution. (I tested this in 7.1.7 but it will work in 7.1.5)

Couple of things you need to know before you start this:

1) What Login Screen are you using

In your login activity you should see a step like this.

This will load the challenge stream defined in your AuthService in the custom tab in the "Credential Challenge Stream".

If blank, like above,  it's using Web-Login.  (You are probably using Web-Login or a customized version of it as it's always expected it will be modified for application/company branding)

2) The ruleset, available to unauthenticated users, you can make changes in.

You need to have a ruleset available to users before they are authenticated to save changes into.

You probably have this setup already. Check your Data-Admin-Requestor  <SystemName>.Browser instance,, SysAdmin->Requestor Type,  and make sure it contains an AccessGroup that provides access to a ruleset used for SSO customizations. This is the AccessGroup used prior to authentication, not the AccessGroup the user will use once authenticated.

My Example:

So this AccessGroup is references a product called SSOAuth that has one ruleset SSOAuth.  (I don't give unauthenticated users access to a full application)

My Solution:

1) Copy Web-Login or your current Login HTML if you have already have something custom.  (If you have upgraded from a older PRPC version you may want to start with Web-Login)

    I called mine WebLoginSSOSnapStart

2) Find the HTML form, will look like this:

<pega:choose>

    <pega:when test="DisableAutoComplete" >

        <form name="main" method="post" onSubmit="return sendLoginRequestForm(event);" action="<pega:reference name="pxThread.pxReqURI" mode="normal" />" target="_top" novalidate="novalidate" autocomplete="off" >

    </pega:when>

    <pega:otherwise>

        <form name="main" method="post" onSubmit="return sendLoginRequestForm(event);" action="<pega:reference name="pxThread.pxReqURI" mode="normal" />" novalidate="novalidate" target="_top" >

    </pega:otherwise>

</pega:choose>

3) Change to:

<%

  String actionURL = tools.findPage("pxThread").getString("pxReqURI");

  String SSOSnapStartQuery = tools.findPage("pxThread").getString("SSOSnapStartQuery");

  if (SSOSnapStartQuery != null && SSOSnapStartQuery.length() > 0)  {

      actionURL += "?" + SSOSnapStartQuery;

  }

  tools.putParamValue("actionURL", com.pega.pegarules.pub.util.StringUtils.urlCrossScriptingFilter(actionURL));

%>

<pega:choose>

    <pega:when test="DisableAutoComplete" >

      <form name="main" method="post" onSubmit="return sendLoginRequestForm(event);" action="<pega:reference name="param.actionURL" mode="normal" />" target="_top" novalidate="novalidate" autocomplete="off" >

    </pega:when>

    <pega:otherwise>

      <form name="main" method="post" onSubmit="return sendLoginRequestForm(event);" action="<pega:reference name="param.actionURL" mode="normal" />" novalidate="novalidate" target="_top" >

    </pega:otherwise>

</pega:choose>

4) Update your Login activity:

    In the pages and class add page pxThread with class reference of Code-Pega-Thread.

    Now you need to get the pyQueryString to the clipboard so it can be displayed in the login HTML:

Note: When you add the Property-Set step for "SSOSnapStartQuery" on page pxThread add the property at the same time in the same ruleset the activity is in.

       Add the pre step condition as so:

     We only want to set SSOSnapStartQuery property if pyQueryString has a value and if SSOSnapStartQuery has not already been set.

5) Update you Data-Admin-AuthService to reference to new HTML screen.

Next Steps:

I realize this is not exactly that easy . I usually I do this type of work on a screen share and explain as I go. If you have problems we can do a screen share.