Thank for the answer, but it's fine I found the issue, I had to use :
# Create IP Config and front-end port
$pip = Get-AzPublicIPAddress -ResourceGroupName APPGATEWAY-RG -Name "appGwPublicFrontendIpIPv4"
$fipconfig = ADD-AzApplicationGatewayFrontendIPConfig `
-Name myAGFrontendIPConfig `
-PublicIPAddress $pip
# Save the application gateway with the configuration changes
Set-AzApplicationGateway -ApplicationGateway $getgw
$frontendportHTTP = Add-AzApplicationGatewayFrontendPort `
-Name myfrontendportHTTP `
-Port 80
$Appgw = Add-AzApplicationGatewayHttpListener `
-ApplicationGateway $getgw `
-Name "QAwebCandidateListenersHttp" `
-Protocol "Http" `
-FrontendIpConfiguration $fipconfig `
-FrontendPort $frontendportHTTP `
-HostName "irap-qacandidat.epsi-inc.com"
# Save the application gateway with the configuration changes
Set-AzApplicationGateway -ApplicationGateway $getg
instead of the first one