Varnish custom backend timeout configuration

We’d like to configure a Varnish backend with custom. connect_timeout, first_byte_timeout and between_bytes_timeout settings; eg. something like this in the .platform/varnish.vcl config:

backend main {
.host = “app”;
.port = “http”;
.connect_timeout = 60s;
.first_byte_timeout = 120s;
.between_bytes_timeout = 120s;

This however seems not work (presumably it conflicts with the automatically generated backend configuration as noted in the docs?) - is there another way.