Setting Up Source NAT IP Address

1. Create an address object with the server IP address

2. To use SNAT create an IPOOL type overload

3. Create a firewall policy and in the destination interface chose the wan interface which will be routing the traffic to the sever IP you can check the interface using  the below command 

get router info routing-table details <server ip>
4. Destinations address should be the one we created in the step 1 

5 Source address and interface you can decide based on your requirement

6 Enable NAT and use the IPpool 

7 You can apply security policies if you want

If the access is for port 443 then can select the https service otherwise you can create a service with the custom port number

I am jot sure what URL you have but if that is server FQDN then you can create an address type FQDN in the step number 1. 
If you already have a DNS server configured then you may not need DNS provided the DNS is able to resolve the server FQDN

1. If it is a server FQDN you need to add it to the destination address in the firewall policy.

2.An IP pool should be created with the source NAT IP address. You can follow this link and use the overload option mentioned in this: https://community.fortinet.com/t5/FortiGate/Technical-Tip-How-to-configure-SNAT-with-IP-pool/ta-p/195322

This IP pool then can be used in the firewall policy to NAT the traffic