Hello @lienaky7273 !
As of right now, there is no access to the App Center on the Designer Platform. A primary reason for this is that the Designer platform is intended to be white-labeled, and there's not currently a way for us to maintain white-labeling in the App Center. It's something we'd love to be able to make available to our Designer customers, but there are a few complications involved.
There is an existing suggestion in our Vote on Features section to which you can add your vote. Here's the suggestion: https://community.weebly.com/t5/Vote-on-Features/Enhancing-our-Service-Offerings-The-lack-of-the-App...
This discussion has been ongoing for some time now (on separate threads too going back to May). When is this going to be available? Do we at least have a time frame on when we might be able to see this done? Surely this can be added and then diasbled / enabled as per other options through the designer dashboard - then it's up to the designer as to whether they want it to be white labeled or not?
It's kind of restrive at the moment and would certainly help people add apps instead of building these things from the ground up...