I made my own weapon mod in my mod, So I made flamethrower and incendiary weapon that unleashing fire, But I don't know how to code them.
Here's a example pictures.

Flame Thrower: Incendiary Weapon: So I need expert who to know the coding to tell me how to make those weapons via coding.
(Sorry for bad English)

For the flamethrower, you'll need a "fire projectile" and a "flame" entity. The projectile will travel, play an animated sprite and eventually disappear. But when it hits an entity inheriting from CBaseMonster (or something like func_breakable), then it will spawn little flames over them, which will continue to deal damage over time until they extinguish.

You will need to know C++ and plenty of the HL SDK to achieve this. If you just want some code, ready to be copy-pasted into your mod, nobody can help you with that, because 1) that is concrete work, not just "helping", and you'll likely need to pay someone for that kind of time, 2) the code may not match your intended design or vision for the weapons.