The present disclosure relates to a supplementary device (1) configured to be mounted to a medicament delivery device (1), wherein the supplementary device comprises a casing (3), a button (5) configured to be received by the casing (3) and configured to actuate a trigger (6) of a medicament delivery device (2), wherein the button (5) is configured to be displaceable from an initial retracted position to an extended position relative to the casing (3), and a button locking member (7) configured to lock the button (5) in the retracted position of the button (5) to prevent displacement of the button (5) from the retracted position to the extended position, wherein the button locking member (7) is configured to release button (5) when the supplementary device (1) is mounted to a medicament delivery device (2), thereby enabling displacement of the button (5) from the retracted position to the extended position.
