Just selecting the text then running the action. The BTT ChatGPT action is "Transform and Replace Selection with ChatGPT" - BTT copies the text you selected and passes it to ChatGPT (along with the ChatGPT System Prompt and ChatGPT User Prompt, if configured in the action), and replaces the selected text with whatever ChatGPT returns.
Yesterday I raised a feature request for an update to the action which could handle other cases, like processing text from the clipboard and/or putting the response into the clipboard rather than pasting it instantly as it does now. But for now it always pastes the response.
Note that all that the "Transform & Replace Selection with ChatGPT" action actually does to 'replace the text' is send Cmd-V to paste it, so if you wanted it to not delete your existing text you could achieve that by deselecting the text before BTT returns, so that the text is pasted after it rather than replacing it, like in this example:
(This only works because it takes ChatGPT a few moments to send a response - if it came back quicker you'd likely not have time to deselect the text before it got replaced.)