Hi,
You can't set it via the portal (not yet, coming soon), but you can using powershell commands in Azure Cloud Shell, see sample below:
$webapp = Get-AzWebApp -Name myWebApp -ResourceGroupName myResourceGroup
$webapp.SiteConfig.minTlsVersion = 1.3
$webapp|Set-AzWebApp
After making the change you may verify minimum TLS using below:
https://www.ssllabs.com/ssltest/analyze.html
Once the test is complete, scroll down to Configuration section of the results, and confirm that TLS 1.3 = Yes and TLS 1.2 = No, indicating that TLS 1.3 is minimum required, as shown below:
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP