With ADHD, focus responds far better to external structure than to willpower. Techniques that reliably help include short timed sprints so the finish line is always in sight, working on one visible task at a time, and making the very first step tiny so starting is almost effortless. A timer that counts down in front of you provides urgency and a clear boundary.
Body doubling, working alongside someone else in person or on a call, is one of the most effective ADHD strategies. So is removing friction in advance: phone in another room, tabs closed, materials ready. Reward yourself for showing up, not just for finishing, since starting is the hard part.
This is general information, not medical advice. If ADHD is significantly affecting your life, a qualified professional can help with diagnosis and treatment.