It is not possible to choose which access point devices connect to on the main AmpliFi network. The benefit of the AmpliFi system and mesh systems as a whole is that it will automatically move devices over to the correct access point.
However, it is possible to create an additional SSID that will only be broadcast to an access point of your choosing and connect the devices to that network. To create an additional SSID follow the article here.
Moreover, if it is desired to have devices connect to the main router over the access points then you can enable an option called "router steering." This option will push devices to connect to the main router if possible. Look at this article to find out more.