Equivalent of Campaign Merge field for HTTP Post?

We send text messages from a campaign using an HTTPS post. We’d like to be able to set the text of the message as a parameter for the campaign, using something like a Campaign merge field (https://help.infusionsoft.com/userguides/campaigns-and-broadcasts/campaign-builder-in-depth/campaign-merge-fields ).

However the HTTPS post configuration doesn’t give the option of Campaign merge fields, and entering ~Campaign.MergeFieldName~ doesn’t work either.

What could we use as a configurable field - that’s not specific to a contact - instead?