Plumbing – Adapter for NTC boiler probe

You don't offer much detail in your question, so it's difficult to determine exactly what you want to do. If you're just looking for manual control you could just energize the G terminal on the furnace, which will run the fan in high speed mode (if the furnace has a variable speed motor). Most thermostats have a switch labeled AUTO and FAN. If you put the switch in the FAN position, the G terminal of the thermostat will be connected to the R terminal (meaning the G terminal is energized).

If you want automatic control, you might be able to use a thermostat that has multiple heating controls. Then you might be able to use the W2 terminal of the thermostat to turn on the furnace fan, and the outdoor boiler.

Again, without more information it's difficult to tell you exactly what you can/should do.

In situations like this, I generally find that the manufacturer is a more reliable source of accurate information than an installer, especially an installer whom you are not using (but then again, you might be wise not to use them, too.) Some installers learned "a particular way" which is either outdated for current equipment or was simply wrong in the first place and depend on that in exclusion to actually reading the instructions; this may not work out well.

The manufacturer has considerably more interest in providing accurate information for successful deployment of their products.