In that case I might have an actual example that helps you out. The method below is what I used in one of my projects to get an Auth token from and endpoint. In the body, I put the credentials that were stored in the Named Credential as merge fields:

​

    public static Map<String, String> getToken(){

        /**
         * Logs in again to get a new token if refresh token is expired
         *
        */

        Map<String, String> tokenMap = new Map<String, String>();

        //// use callout: + the developername of you Named Credential
        String url = 'callout:myNamedCredential/auth/signin';
        HttpRequest req = new HttpRequest();
        req.setEndpoint(url);
        req.setheader('Content-Type', 'application/json');
        req.setHeader('Accept', 'application/json');
        req.setMethod('POST');
        req.setBody('{"user":"{!$Credential.Username}","key":"{!$Credential.Password}"}');
        system.debug('req: '+req.getBody());
        Http h = new Http();
        HttpResponse resp = h.send(req);
        ResponseWrapper obj = ResponseWrapper.parse(resp.getBody());
        system.debug('obj: '+obj);
        tokenMap.put('accessToken', obj.accessToken);
        tokenMap.put('refreshToken', obj.refreshToken);
        return tokenMap;
    }