Try it in the GUI's console first:

• Go to "Window->Console" and select "wallet: (none)" (not required)
• Then type: loadwallet "wallet_name" true

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In the command line, use bitcoin-cli with the same command as the above:

• Use Windows Explorer and go to daemon/bin folder inside bitcoin's install directory (default: "C:\Program Files\Bitcoin\daemon")
• Click "File->Open Windows Powershell".
• In Poweshell, type the command: ./bitcoin-cli loadwallet  "wallet_name" true

But in order for bitcoin-cli to work with bitcoin-qt, you must use --server arg with your bitcoin-qt or server=1/ in 'bitcoin.conf' file.
In case you've set a custom data directory and not using rpc username and password, --datadir= should also be indicated for it to find the ".cookie" file in that directory.

You can change "true" to "false" if you do not want to keep the wallet loaded in the next session. (not recommended if 'prune block storage' is enabled)