How may I assign static listen addresses to the admin webpage ?

You should only allow webadmin access from dedicated hosts.

Not the target IP but the source IP should be decisive.

Thanks for your reply. How can I do it ?

Within administration / device access select only trusted zones within "Local service ACL / HTTP"

... or more restrictive ... select only trusted hosts within "Local service ACL exception rule" and allow HTTP access.

but be careful ... first allow / test / forbid ... do not forbid your own access.

Thank you for your reply. I can not yet find the option for only trusted zones within "Local service ACL / HTTP". This is my interface. Where may I find it ?

HTTPS is Webadmin.

But based on your screenshot, you are using the same Zone for every Interface. So you cannot limit it based on Zones. 

Well, I have created multiple VLAN interfaces and mostly all of them have been put under the LAN zones. Has this been un-wise for me to do so ? If so, how can I now limit it without having to rely on Zones ?

You can use one Zone, but you will lack in granulate later. With Zone based concepts and VLANs in own Zones, you can define more granular firewall rules in a quicker manner. 

Then what can I do now to achieve the question in the title of this topic ? And what else can I do later on to re-gain your ‘granulate’ !?

Then what can I do now to achieve the question in the title of this topic ? And what else can I do later on to re-gain your ‘granulate’ !?