After logging in your instance, on the Ribbon, click Settings and then click Customizations.
In the Customizations page, click Developer Resources.
Once you are in the Developer Resources page, look for a section called Instance Web API.
With the Instance Web API, you can call a certain entity type, for example:
View all other entities
var get_client_url = parent.Xrm.Page.context.getClientUrl();
var req = new XMLHttpRequest();
req.open("GET", get_client_url + "/api/data/v9.1/tasks", false);
req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
var result = JSON.parse(req.responseText);
Microsoft. (2019, March 21). Web API Reference. Retrieved from Microsoft Docs: https://docs.microsoft.com/en-us/dynamics365/customer-engagement/web-api/about?view=dynamics-ce-odata-9