Spring Boot SAML2 add custom parameter to idp url

Question

I need to add an extra parameter called "locale" to the idp url. The url should look like this : https://idpURL?SAMLRequest=...&RelayState=...&SigAlg=...&Signature=...&Locale=DE

I've already done a project that uses the idp to connect. Now I'd like to add this parameter.

    @Bean
public SecurityFilterChain filterChain(final HttpSecurity http,
                                       final RelyingPartyRegistrationRepositoryImpl rpRepo,
                                       final SamlResponseAuthenticationConverter authConverter) throws Exception {
    RelyingPartyResolver relyingPartyRegistrationResolver = new RelyingPartyResolver(rpRepo);
    OpenSaml4AuthenticationRequestResolver authenticationRequestResolver = new OpenSaml4AuthenticationRequestResolver(relyingPartyRegistrationResolver);
    authenticationRequestResolver.setRequestMatcher(new AntPathRequestMatcher("/saml/login"));
    authenticationRequestResolver.setAuthnRequestCustomizer(authnRequestCustomizer());

    OpenSaml4AuthenticationProvider authenticationProvider = new OpenSaml4AuthenticationProvider();
    authenticationProvider.setResponseAuthenticationConverter(authConverter);

    http.saml2Login(samlLogin ->
            samlLogin
                    .successHandler(samlSuccessRedirectHandler())
                    .authenticationManager(new ProviderManager(authenticationProvider))
                    .authenticationRequestResolver(authenticationRequestResolver)
                    .authenticationConverter(new Saml2AuthenticationTokenConverter(relyingPartyRegistrationResolver))
                    .loginProcessingUrl("/saml/SSO"));

    return http.build();
}

/**
 * @return Options used to build the outgoing SAML authentication requests.
 */
@Bean
protected Consumer<OpenSaml4AuthenticationRequestResolver.AuthnRequestContext> authnRequestCustomizer() {
    return (c) -> {
        final AuthnRequest authnRequest = c.getAuthnRequest();

        NameIDPolicy nameIdPolicy = (NameIDPolicy) XMLObjectSupport.buildXMLObject(NameIDPolicy.DEFAULT_ELEMENT_NAME);
        nameIdPolicy.setAllowCreate(true);
        nameIdPolicy.setFormat("urn:oasis:names:tc:SAML:2.0:nameid-format:transient");
        authnRequest.setNameIDPolicy(nameIdPolicy);

        authnRequest.setForceAuthn(true);

        RequestedAuthnContext requestedAuthnContext = (RequestedAuthnContext) XMLObjectSupport.buildXMLObject(RequestedAuthnContext.DEFAULT_ELEMENT_NAME);
        requestedAuthnContext.setComparison(AuthnContextComparisonTypeEnumeration.EXACT);

        AuthnContextClassRef authnContextClassRef = (AuthnContextClassRef) XMLObjectSupport.buildXMLObject(AuthnContextClassRef.DEFAULT_ELEMENT_NAME);
        authnContextClassRef.setAuthnContextClassRef(AUTHN_CONTEXT_CLASS_REF);

        requestedAuthnContext.getAuthnContextClassRefs().add(authnContextClassRef);


        authnRequest.setRequestedAuthnContext(requestedAuthnContext);
    };
}

@Bean
SamlSavedRequestAwareAuthenticationSuccessHandler samlSuccessRedirectHandler() {
    return new SamlSavedRequestAwareAuthenticationSuccessHandler()();
}

The Saml2WebSsoAuthenticationRequestFilter (from package org.springframework.security.saml2.provider.service.web) takes care of adding various parameters to the idp url via the method. Would it be possible to force it to add other parameters? Is there another way of adding a parameter to this url?