import okhttp3.OkHttpClient;

import okhttp3.FormBody;

import okhttp3.Request;

import okhttp3.RequestBody;

// HashMap with Params

HashMap<String, String> params = new HashMap<>();

params.put( "Param1", "A" );

params.put( "Param2", "B" );

// Initialize Builder (not RequestBody)

FormBody.Builder builder = new FormBody.Builder();

// Add Params to Builder

for ( Map.Entry<String, String> entry : params.entrySet() ) {

    builder.add( entry.getKey(), entry.getValue() );

}

// Create RequestBody

RequestBody formBody = builder.build();

// Create Request (same)

Request request = new Request.Builder()

        .url( "url" )

        .post( formBody )

        .build();