To enable the new logon mechanism you have to change your web.config in the following way:

(1) disable forms auth by commenting it out:

<!--
    <authentication mode="Forms">
      <forms name="MyFormsAuthCookie" timeout="1440" slidingExpiration="true" loginUrl="Logon.aspx" ticketCompatibilityMode="Framework40"/>
    </authentication>
-->


(2) enable AD FS OIDC support by specifying an AdFsServer:

Given that you have the following AD FS configuration...

and the following AD FS Application Group for DPE...

The DPE web.config would look like...

<add key="AdFsServer" value="https://dpe2019.davidsystems.com/adfs"/>

<add key="ida:AdfsMetadataEndpoint" value="https://dpe2019.davidsystems.com/federationmetadata/2007-06/federationmetadata.xml" />
<add key="ida:Audience" value="microsoft:identityserver:jsDPE" />
<add key="ida:Issuer" value="http://dpe2019.davidsystems.com/adfs/services/trust" />


(3) enable WCF service support for OIDC by adding oidcEndpoint behavior extension and reference it in <behavior name="cors">:

<system.serviceModel>
...
    <behaviors>
      <endpointBehaviors>
        ...
        <behavior name="cors">
          <webHttp/>
          <jsonWebHttp/>
          <enableCorsEndpoint/>
          <oidcEndpoint/>
        </behavior>
      </endpointBehaviors>
    <behaviors>
...
    <extensions>
    ...
      </behaviorExtensions>
        ...
        <add name="oidcEndpoint" type="David.Dpe.Wcf.OidcEndpointBehaviorElement, David.Dpe.Wcf"/>
        ...
      </behaviorExtensions>
    </extensions>   
...
</system.serviceModel>