@Stemina SC This is not specific to only APIM but how you can call any service with basic authentication. As per IETF RFC 7617 described here you need to pass the Authorization
as basic
type along with the encrypted username and password.
Postman does have the option and looks like it will do encryption for you while calling any API and it will add the Authorization header for you.
In other tools for example as if your user-id "Aladdin" and password "open sesame" then you need to pass the encrypted base64 string in the format username : password
as mentioned in IETF RFC 7617. You can leverage the online tools to encrypt or decrypt the string or write your own code if you don't want to post the string (username:password) to any site over the internet.
You can add the header as below in your tool. To get more familiar with postman you can refer to postman document.
Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==