How to use Gitlab for get nofication from other custom service

Hello. Recently I have started to move the CI-CD process of my project to GitLab. Here, we have a requirement.
Suppose, we have to deploy our build through Git pipeline in 3 environments i.e. DevEnv, PreProdEnv, and ProdEnv. Once, the merge request is completed and code is successfully merged, the build should be deployed properly in DevEnv without any outer intervention. But, if developer wants to deploy the same build in PreProd or Prod Env, the Git pipeline should first call a Groovy or NodeJs service that will send a notification to concerned project manager. If project manager approves it by triggering another Groovy or NodeJs service, then this service should trigger the Git pipeline to deploy the build in PreProd or Prod Env.

I searched in GitL