You can use Microsoft Graph API endpoint https://graph.microsoft.com/v1.0/users
and use $filter query parameter to get list of users which meets the filter criteria. For example - To get users having display name starts with "Joshua Chung", the syntax is as follows:
GET https://graph.microsoft.com/v1.0/users?$filter=startswith(displayName,'Joshua Chung')
Please refer to the documentation to know more about using Microsoft Graph Users endpoint. Also, refer to documentation to learn more about Query parameters.
You can also refer to Microsoft Graph auth overview to get the authorization tokens and Use the Microsoft Graph API to use tokens for calling Microsoft Graph REST API's. You can also use Microsoft Graph Explorer
to test requests using Microsoft Graph APIs.