"""Encodes specified values in a dictionary using base64 encoding.

The function takes a dictionary and a list of keys. For each key in the list, the corresponding value is JSON-serialized, base64-encoded, and then modified to be URL-safe. The encoded value is prefixed with "b64_" and stored in a new dictionary.

Args:
    decoded_map (dict): The dictionary containing key-value pairs to be processed.
    apply_on_keys (list): A list of keys whose corresponding values should be encoded.

Returns:
    dict: A new dictionary with the specified values encoded and prefixed with "b64_".

Note:
    The base64 encoding is manually adjusted to be URL-safe by replacing '+' with '-', '/' with '_', and stripping trailing '=' characters.
"""
You should write code starting with:
import base64
import json

def encode_url_params(decoded_map: dict, apply_on_keys: list = None) -> dict:
    # TODO: Implement this function
    pass

