setClientCertificateName() can be used to assign certificate when making Salesforce Apex Callouts.

Sample Certificate:

Sample Code:

HttpRequest req = new HttpRequest();
req.setClientCertificateName( 'Test_Certificate' );
req.setMethod ( 'GET' );
System.debug(
    'Request is ' +
    req
);

So, when doing Apex Callouts, use setClientCertificateName() method to assign the certificate.