You should create library variables with the values you want to set:
https://docs.microsoft.com/en-us/azure/devops/pipelines/library/variable-groups?view=azure-devops&tabs=yaml
After that, you'll be able to retrieve the values by using:
$(customer)
I recommend you to store the content of the secrets into Azure Key Vault, and retrieve the secrets from there (also using library variables).
Lastly, all you need to do is get those values and set your app service settings (I do this through a powershell task):
$myResourceGroup = 'PartsUnlimitedMRP'
$mySite = 'centpartsunlimited'
$webApp = Get-AzureRMWebAppSlot -ResourceGroupName $myResourceGroup -Name $mySite -Slot production
$appSettingList = $webApp.SiteConfig.AppSettings
$hash = @{}
ForEach ($kvp in $appSettingList) {
$hash[$kvp.Name] = $kvp.Value
}
$hash['customer'] = $(customer)
Set-AzureRMWebAppSlot -ResourceGroupName $myResourceGroup -Name $mySite -AppSettings $hash -Slot production