I think since Keycloak 18 it's no longer possible to upload scripts in the admin console, I think for security reasons.

That said it can be tedious to develop a script in this way. For development testing, on your development Keycloak instance, you can enable the oidc-script-based-protocol-mapper.

• The scripts feature needs to be enabled
• Create a oidc-script-based-protocol-mapper.jar with the following content in META-INF/services/org.keycloak.protocol.ProtocolMapper

  org.keycloak.protocol.oidc.mappers.ScriptBasedOIDCProtocolMapper
  

• Deploy to providers
• When you login to your master realm under Provider Info under protocol-mapper you should see oidc-script-based-protocol-mapper as one of them

You should now be able to Add mapper > By configuration > Script Mapper.

For the script, the return result that you set on exports will be the claim value used. The claim name is configured on the mapper as Token Claim Name, this is the same if you deployed a script.

The following is a sample script that sets the claim values to the client role mappings of that client.

var ArrayList = Java.type("java.util.ArrayList");
var claimValues = new ArrayList();
var client = keycloakSession.getContext().getClient();
var forEach = Array.prototype.forEach;
forEach.call(user.getClientRoleMappingsStream(client).toArray(), function(roleModel) {
  claimValues.add(roleModel.getName());
});
exports = claimValues;

• Multivalued: On

You can set it to a JSON String

exports = "{ \"hello\": \"world\" }";

• Claim JSON Type: JSON

That said since you have access to the token variable you can also just manipulate it

token.setAcr("claimValue");
token.getOtherClaims().put("claimName", "claimValue");

You can't seem to edit the script by clicking Save right after the first creation for some reason (it says Could not create mapping: 'Protocol mapper exists with same name'), but if you back out and come back in you seem to be able to edit the mapper.

Also when evaluating the outputs in Clients > Client details > Client scopes > Evaluate, I notice that you need to change the Users to see the token result when you update the mappers.