Connect to the WiFi of the smartphone and set the proxy

Is it possible with OpenWrt to connect to the WiFi of the smartphone and also to set the proxy?

The long story:
I have a particular use case where the router must take the internet connection via WiFi from a smartphone connected in turn to another WiFi network. Basically the smartphone acts as a WiFi repeater.

Unfortunately, the only way I have found to make this smartphone to act as a repeater is with an app that creates a hotspot on the smartphone with its SSID and password but in addition to these it requires that the client also set up a proxy that is the smartphone itself.

external_wifi <-- smartphone + WiFi hotspot <-- OpenWrt (must setup a proxy to the smartphone address) <-- internal_lan_devices