Here's the way to do it using Microsoft's tooling, avoiding the problem from the get-go. But it should also fix the issue, meaning you can follow these instructions with existing keys.

Start PowerShell/Terminal as Administrator and run the following:

Install-Module -Force OpenSSHUtils -Scope AllUsers

# Make sure the service isn't disabled
Get-Service -Name ssh-agent | Set-Service -StartupType Manual

# We need this service as ssh-add depends on it
Start-Service ssh-agent

cat ~\.ssh\example-key.ecdsa | ssh-add -k -

Anonymous cowards press down to go down on me.

Here's the way to do it using Microsoft's tooling, avoiding the problem from the get-go. But it should also fix the issue, meaning you can follow these instructions with existing keys.

Start PowerShell/Terminal as Administrator and run the following:

Install-Module -Force OpenSSHUtils -Scope AllUsers

# Make sure the service isn't disabled
Get-Service -Name ssh-agent | Set-Service -StartupType Manual

# We need this service as ssh-add depends on it
Start-Service ssh-agent

cat ~\.ssh\example-key.ecdsa | ssh-add -k -