No, you did not echo the entire %1. As I can see, =Dq3V-KdKslU is stripped.

The issue is the character =. To avoid stripping, you can enter this argument in quotation marks:

> mp3.bat "https://www.youtube.com/watch?v=Dq3V-KdKslU"

Then the entire argument string will be passed. In your echo, you will see

Argument 1: https://www.youtube.com/watch?v=Dq3V-KdKslU

This value will be correctly passed to yt-dlp and elsewhere.