Article applies to: Scarlett 6i6, 8i6,18i6,18i8,18i20. Saffire - All models except Saffire 6 usb. Clarett - All models.
Pro Tools has a feature called a "Hardware Insert" where you can use a piece of Outboard gear similar to a plug in, in the Insert Section of your Channel.
Pro Tools will only use a Hardware Insert with the same Input and Output number, so you will likely want to use a channel 3, 4, 5, or 6, as 1/2 Outputs are likely your Studio Monitors, and 7/8 and 9/10 are sent to the Headphones so you will want to leave these for the Main Mix as well.
In this example we will use Channel 3.
First in Pro Tools go to Setup > I/O and make sure your Hardware Inserts are set up. In the screenshot below you will see a named Insert for 3/4 Inputs "Lexicon" for a 2 channel Lexicon unit to be connected on Inputs 3/4.
Then in the Insert area of your Track you can add one or both channels. The session only has a Mono Audio Channel to be processed, so you will just see "Lexicon.L" which is the Left Channel plugged into Input 3. Pro Tools also allows you to use the drop down triangles to name the individual channels differently if you prefer, rather than in pairs.
Then in Mix Control you need to route these channels Out of the DAW's Line Outs to the Physical Line Outs. So you would send DAW 3 > Output 3 and DAW 4 > Output 4.
Also, if using a Pro 40, Liquid Saffire 56 or Scarlett 18i20, be sure to Shift+Click on the Monitor Enable Buttons for Channels 3 and 4, just above the Monitor Knob on MixControl. This unlinks the desired channels from that large Monitor Knob and sends a full Line Level volume. Always be careful not to shift click if gear such as speakers are plugged in, or you aren't 100% sure yet about your routing, as in some cases, say if you had a Mix containing Input 3 sent to Output 3, you can create a sudden feedback loop which can be VERY loud.
You have now created a Hardware Insert on 3, and have 4 available for another channel in your session as well!