First of all you need to enable console by navigating Settings->Mouse & Keyboard->Advanced
Then all you have to do is typing: map and any of the console commands like the following example:
''map nameofthemap''

Fun Fact: You can play the Half-Life flawlessly in the ''Opposing Force" & "Blue Shift" expansion packs by entering the campaign commands to the console.
This feature does not work anymore, it has probably been glitched with the 25th anniversary update and the compatibility of half-life mods with the original game.

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Black Mesa Inbound (Chapter 1)
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Anomalous Materials (Chapter 2)
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Note: c1a0c is part of Chapter 3: Unforeseen Consequences

Unforeseen Consequences (Chapter 3)
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Note: c1a1e does not exist in game files

Office Complex (Chapter 4)
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"We've got Hostiles" (Chapter 5)
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Blast Pit (Chapter 6)
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Note: c1a4a, c1a4c, and c1a4h don't exist in game files.

Power Up (Chapter 7)
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On a rail (Chapter 8)
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Apprehension (Chapter 9)
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Residue Processing (Chapter 10)
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Questionable Ethics (Chapter 11)
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Surface Tension (Chapter 12)
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Note: c2a5h through c2a5v don't exist in game files.

"Forget about Freeman!" (Chapter 13)
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Lambda Core (Chapter 14)
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Xen (Chapter 15)
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Gonarch's Lair (Chapter 16)
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Interloper (Chapter 17)
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Nihilanth (Chapter 18)
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Endgame (Chapter 19)
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Boot Camp: boot_camp

Bounce: bounce

Datacore: datacore

Lambda Bunker: lamda_bunker

Snark Pit: snark_pit

Stalkyard: stalkyard

Subtransit: subtransit

Undertow: undertow

Frenzy: frenzy

As of 17th November, 2023, VALVe has added 4 brand-new multiplayer maps, plus three maps originally included solely for the Half-Life: Further Data retail CD.

Contamination: contamination

Pool Party: pool_party

Disposal: disposal

Rocket Frenzy: rocket_frenzy

Double Cross: doublecross

Rust Mill: rustmill

Xen DM: xen_dm