You don't really need your own implementation of the FeignRequestInterceptor as there is already BasicAuthRequestInterceptor in the feign.auth package that does exactly the same.

That being said, you basically have almost everything set up already. All is left to do is to define the basicAuthRequestInterceptor bean with specific username and password:

@Bean
public RequestInterceptor basicAuthRequestInterceptor() {
    return new BasicAuthRequestInterceptor("username", "password");
}

Thanks to this comment, I managed to tidy up my implementation a little bit. So no more need for specifying encode/decoder nonsense, or having to manually build my Feign client.

The docs here provide some info, but as is typical they are a bit thin on concrete examples, so perhaps the below will help someone else. Note: I'm using spring boot, and including feign like so in build.gradle implementation 'org.springframework.cloud:spring-cloud-starter-openfeign'

First, create the RequestInterceptor like so:

import org.springframework.context.annotation.Bean;
import org.springframework.security.core.Authentication;
import org.springframework.security.core.context.SecurityContextHolder;
import org.springframework.security.oauth2.jwt.Jwt;

import feign.RequestInterceptor;
import lombok.extern.slf4j.Slf4j;

/**
 * A Feign configuration to add the incoming Bearer token to an outgoing feign client request.
 * Only annotate this class with "@Configuration" if you want this interceptor to apply globally to all your Feign clients.
 * Otherwise you risk exposing the auth token to a third party, or adding it unnecessarily to requests that don't need it.
 */
@Slf4j
public class BearerAuthFeignConfig {

    @Bean
    public RequestInterceptor bearerAuthRequestInterceptor() {
        return requestTemplate -> {
            Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
            if (authentication != null && authentication.getPrincipal() instanceof Jwt) {
                Jwt jwt = (Jwt) authentication.getPrincipal();
                requestTemplate.header("Authorization", "Bearer " + jwt.getTokenValue());
            } else {
                log.error("Unable to add Authoriation header to Feign requestTemplate");
            }
        };
    }
}

Then when declaring your feign client, pass the configuration

@FeignClient(
    name = "my-client-that-needs-the-auth",
    configuration = BearerAuthFeignConfig.class,
    url = "http://whatever.com"
)
public interface PlayerManagementClient {
  ...

You'll also need the @EnableFeignClients annotation on your @SpringBootApplication class

If your JWT token provider is OAuth 2.0 compliant, you can configure the OAuth2FeignRequestInterceptor, with an OAuth2ProtectedResourceDetails object. This object is the base class for all OAuth 2.0 grant type information. In your case, I recommend using ResourceOwnerPasswordResourceDetails instead. This will allow you to configure an interceptor using a username and password.

@Configuration
public class OAuth2RequestInterceptorConfiguration {

    @Bean
    public OAuth2FeignRequestInterceptor requestInterceptor() {
        OAuth2ClientContext clientContext = new DefaultOAuth2ClientContext();
        OAuth2ProtectedResourceDetails resourceDetails =
            new ResourceOwnerPasswordResourceDetails();
        resourceDetails.setUsername("username");
        resourceDetails.setPassword("password");
        return new OAuth2FeignRequestInterceptor(clientContext, resourceDetails);
    }
}

For other cases, you will need to create your own RequestInterceptor

public class MyRequestInterceptor implements RequestInterceptor {

    private String jwt;
    private LocalDateTime expirationDate;

    @Override
    public void apply(RequestTemplate requestTemplate) {
        /* validate and refresh your token, this sample is not thread safe */
        if (LocalDateTime.now().isAfter(expirationDate)) {
            requestToken();
        }

        /* use the token */
        requestTemplate.header("Authorization: Bearer " + this.jwt);
    }
}