I'm not sure what you mean with "needs a buildscript classpath". Are you trying to use a dependency in your convention plugin? If so you should add it to build.gradle in the buildSrc directory.

An example of what you are trying to do would help a lot.

I threw this together really quick: https://github.com/foip/jib-convention-test . The crux is adding the jib plugin and the extension to buildSrc/build.gradle.kts. I don't know if this matches your project setup, so let me know if this does or doesn't work for you (:

Edit:

To put the answer to the original question in more general terms for anyone who stumbles upon this thread: In this case the jib-gradle-plugin is applied as a plugin in the root build.gradle.kts, but it needs extra runtime dependencies for its extensions. You normally would declare those dependencies like so:

// in root build.gradle.kts
buildscript {
  dependencies {
    classpath(/* dependency */)
  }
}

When you want to write a convention plugin to wrap that configuration, basically everything that went inside that buildscript block now goes into buildSrc/build.gradle.kts:

// in buildSrc/build.gradle.kts
repositories {
  mavenCentral()
}

dependencies {
  implementation(/* dependency */)
}

That goes for other dependencies as well, for example if you want to use a library to write a custom task, and then you refactor that task into a plugin inbuildSrc.