The goal is simulate the idea "You are hearing but not listening". My first implementation idea is using subtitle on specific sentences/keywords to show what a character is actually listening but many people don't like subtitle.
Unless you are shooting for some avant-garde cinematic experience which immerses the viewer in the character's sensoria, the actor is the vehicle to communicate the impacts and experience of their defining mental and emotional traits.
If this was prose, you'd have insight into the character's internal state to communicate the character's experience. For visual media, since you are restricted to dialogue and action, you craft the character's reactions and dialogue to demonstrate that they are not fully following the arc of the moment. You also can show other characters' reactions to your POV character's dialogue.
It would be important that the viewer is able to fully understand the events in the scene. This way the viewer is able to draw upon their own experiences to decide that this POV character is experiencing an attention deficit versus something else like dementia, self-absorption, or generally not paying attention.
It could also be a challenge to pinpoint this conclusion for viewers who don't have the prerequisite knowledge regarding attention-deficit conditions. Maybe brief exposition might help with that. Kind of depends on the story.
The strongest questions for me, personally, are: do people experiencing attention deficit experience it as frustration or confusion with themselves or with others, or are they unaware that they are not tracking with the same concerns as others in a conversation.
If you're looking to show a character with ADHD, I would recommend listening to how ADHD people describe their experiences. A lot of how it is described and perceived is from frustrated non-neurodiverse people, and that includes the DSM.
If you're looking to just have one character distracted while another is talking, both body language and dialogue can easily portray this. Eye contact is intermittent or 100% unwavering (like the character has zoned out or dissociated), no part of the body is turned toward the speaker, etc. In dialogue, you can show that the listener is only catching bits and pieces by completely missing certain important pieces and then reacting oddly to something they do finally catch.