Every matchup has its own permanent URL. The pattern is /compare-pokemon/[first]-vs-[second], so Pikachu vs Raichu lives at /compare-pokemon/pikachu-vs-raichu. Copy it from the address bar.
Step 1
Pick two Pokemon. The URL updates automatically once both slots are filled.
Step 2
Copy the URL from your browser's address bar.
Step 3
Paste anywhere. The recipient opens it and sees the exact same comparison.
Step 4
Old URLs with the ?a= and ?b= query format still work and redirect to the clean path.
Related: What is the Compare Pokemon tool.