Open your HTML email signature file in your web browser.
Press Cmd + A to select the entire email signature, then press Cmd + C to copy it.
IMPORTANT: Do not use your mouse to select the area, only use Cmd + A as you may miss out on grabbing some of the code.
Open eM Client and from your menu bar select eM Client, then Preferences.
On the left menu, expand Mail and select Templates and Signatures. Click the Signatures... button.
Create a New signature by clicking the button at the top left of the window. Enter a name for your email signature, then paste your email signature in the field below by clicking inside and pressing Cmd + V to paste your copied signature. Click Save.
From your Signature section, select the Signature for new mail drop down and select your email signature. Repeat this step for Replies and Forwards below.
Close out of your settings and compose a new email. Your email signature will now automatically insert when sending an email within eM Client for macOS.