In LoR, it is possible to conduct matches with any player, for this you need to add the player as a friend

How to add to friends in Legends of Runeterra

To add to friends you need to know the player’s nickname. You can see the nickname by clicking on the friends icon, then on your avatar.

If you know the nickname of the character. Click on the “+” sign and enter a two-part nickname.

After the player has accepted your invitation to friends, you can challenge him to a duel. How to invite a friend to the match is shown below in the screenshot.

After a friend accepts your offer, all that remains is to select a deck and click Ready.