How can we add HTML to an existing Email Template usign deluge

How can we add HTML to an existing Email Template usign deluge

Is there any way to add HTML text to a CRM Email Template through deluge?
We were thinking to use a multiline field with a text lets say <p>This is my text</p> and put this text in the Email Template through merge field.

But it returns the whole text <p>This is my text</p> in the Email Template.
Same goes if we use the replaceAll() function but getting the Email Template through Get Email Template API, replace something with this <p>This is my text</p> and post it using invokeURL and Send Mail API

As a workaround I was thinking to put this text in a Creator Richtext field and call it from there but I am not sure if it will work.

What other possibilities we have? How can we pass HTML to an existing email template and send it to a client?