Game concept · The Fate of Iberia
Bargain Fealty
The game also links it as Bargained Fealty.
From the game's encyclopedia
This special Interaction is available to Involved Rulers who are in a defensive War.
The War Leader asks a neighboring higher-Rank Ruler to join their war in exchange for becoming their Vassal.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Bargain Fealty is an entry of Crusader Kings III's in-game encyclopedia (The Fate of Iberia). The game explains it: "This special Interaction is available to Involved Rulers who are in a defensive War." The game also links it as Bargained Fealty. Patch 1.20.1.2, from game/common/game_concepts/03_fp2_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/03_fp2_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.