1) Download and install SharePoint Online Management Shell
2) Execute the script below. (Note that the Sideloading Feature ID is harcoded in SharePoint, as are many other OOTB features)
#CODE STARTS HERE $programFiles = [environment]::getfolderpath("programfiles") add-type -Path $programFiles'\SharePoint Online Management Shell\Microsoft.Online.SharePoint.PowerShell\Microsoft.SharePoint.Client.dll' Write-Host 'Ready to enable Sideloading' $siteurl = Read-Host 'Site Url' $username = Read-Host "User Name" $password = Read-Host -AsSecureString 'Password' $outfilepath = $siteurl -replace ':', '_' -replace '/', '_' try { [Microsoft.SharePoint.Client.ClientContext]$cc = New-Object Microsoft.SharePoint.Client.ClientContext($siteurl) [Microsoft.SharePoint.Client.SharePointOnlineCredentials]$spocreds = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $password) $cc.Credentials = $spocreds $site = $cc.Site; $sideLoadingGuid = new-object System.Guid "AE3A1339-61F5-4f8f-81A7-ABD2DA956A7D" $site.Features.Add($sideLoadingGuid, $true, [Microsoft.SharePoint.Client.FeatureDefinitionScope]::None); $cc.ExecuteQuery(); Write-Host -ForegroundColor Green 'SideLoading feature enabled on site' $siteurl #Activate the Developer Site feature } catch { Write-Host -ForegroundColor Red 'Error encountered when trying to enable SideLoading feature' $siteurl, ':' $Error[0].ToString(); } #CODE ENDS HERE