Your going to have to use the console to create these binds, There is no menu to bind specific weapons to your keys

to open the console press the '~' key, its to the left of the one key on your keyboard
(not the one key on your numpad the other one key ((who even has a numpad anymore? everyone I know has a 60% or a ten keyless))

The syntax for binding weapons to keys is:
Bind [key] weapon_[example]

semicolons indicate the end of a command in gold src's console.

this list has semicolons so once you add the keys you want between 'bind' and 'weapon' you can just paste it all into the console and press enter. This way you don't have to alt tab over and over.

make sure there's a space between the key your trying to bind and the rest of the command
bindk weapon_crowbar or bind kweapon_crowbar wont work.

bind weapon_crowbar;
bind weapon_9mmhandgun;
bind weapon_357;
bind weapon_9mmAR;
bind weapon_shotgun;
bind weapon_crossbow;
bind weapon_rpg;
bind weapon_gauss;
bind weapon_egon;
bind weapon_hornetgun;
bind weapon_handgrenade;
bind weapon_satchel;
bind weapon_tripmine;
bind weapon_snark;

Opposing force weapons

bind weapon_eagle;
bind weapon_sniperrifle;
bind weapon_m249;
bind weapon_knife;
bind weapon_sporelauncher;
bind weapon_grapple;
bind weapon_pipewrench;
bind weapon_shockrifle;
bind weapon_displacer;

Heres another useful guide, they don't have as easy of a method for people to bind their keys but they have other useful information
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=424275559

Thanks to Swiftshift, they are where I learned these commands in the first place.
Thanks to forever now… they posted the opposing force weapon names in Swiftshifts’ guides comments

If you have any issues please be thorough when you describe them so I can help you and improve the guide. I cant fix an issue If I don't know what it is, 'Doesn't work' doesn't help

Have fun!