Seamless login from Regular Web App to SPA

Hi, We have two applications and want to seamlessly login from Regular Web App to SPA

1 - Regular web application (angular v1.3.20 + spring-boot v2.x) it uses authentication api to authenticate user
2 - Single Page application (react + spring-boot) it uses auth0-spa-js to authenticate user

I am having difficulty understanding how we can seamlessly login user from Regular web app to SPA. They both share same tenant and db connection.