Dear [Congress Person],
The constitution states that war can only be declared by congress. Launching a nuclear first strike is certainly an act of war. I urge you and members of congress to make clear your necessary role in declaring war and to remove from the president the ability to make unauthorised first strikes.
[The address at which you are registered to vote]
You can fax your senators for free: https://faxzero.com/fax_senate.php. You do not need to create an account, but you do need to enter your email address. You should contact both senators.
If you are a US resident who cannot vote, you can, of course, still contact your local senator and representative. If you cannot vote in the US and do not live in the US, you should contact your local government representative and express your concerns. Especially if you government is a NATO member or allied with the US in some other way, this could be helpful.
There’s already a bill in congress on this topic, the Restricting First Use of Nuclear Weapons act of 2017. Here’s a sample phone script if you want to call congress: https://www.wagingpeace.org/sample-phone-script-restricting-first-use-nuclear-weapons-act/