How to make a Lamb Seekh Kebab more juicy and soft?
There are no "secret ingredients" in making soft and juicy kebab. Much like sausage, meatballs, or meatloaf, this ground meat based food relies on salt, fat, and proper cooking to remain juicy. I recommend Chapter 5 of J. Kenji Lopez-Alt's The Food Lab for an explanation of how these elements interact (though I do not believe the book has a kebab recipe).

With this in mind, find a recipe and follow it! Make sure to include the proper amount of salt (preferably measuring by weight) and to use a sufficiently fatty cut of lamb. Then take care to not overcook the kebab, using a thermometer to check the internal temperature.