Merge these into your build.gradle file to add the blockbench-import-library as dependency.
blockbench-import-library requires certain Polymer modules aswell, which are included here.

repositories {
    maven { url 'https://maven.tomalbrc.de' } // blockbench-import-library
    maven { url 'https://maven.nucleoid.xyz' } // Polymer
}

dependencies {
    modImplementation include("de.tomalbrc:blockbench-import-library:[BIL_VERSION]")

    modImplementation "eu.pb4:polymer-core:[POLYMER_VERSION]"
    modImplementation "eu.pb4:polymer-networking:[POLYMER_VERSION]"
    modImplementation "eu.pb4:polymer-resource-pack:[POLYMER_VERSION]"
    modImplementation "eu.pb4:polymer-virtual-entity:[POLYMER_VERSION]"

    // Useful for automatically handling resourcepacks in dev environment, but not required.
    modRuntimeOnly "eu.pb4:polymer-autohost:[POLYMER_VERSION]"
}

It is important to make sure these dependencies are available at runtime.
You can do this by either adding these modules to your dependencies in fabric.mod.json, or by including them in your jar file as Jar-in-Jar.

"blockbench-import-library": "*",
"polymer-core": "*",
"polymer-networking": "*",
"polymer-virtual-entity": "*",
"polymer-resource-pack": "*"