@Patil, Nikita [ON CONTRACT] Thanks for reaching out. From the description i understand that you wanted to navigate from Page: API's list to a built custom page when you click a particular api correct me if i am wrong here
To do that open the developer portal in an admin mode, go to APIs page click on edit widget as shown below
click on hyper link image add your url to point custom page instead of API details page and save the changes
do let me know incase of further queries, I would be happy to assist you.