I ended up doing the following things (considering that the instance has Integration Hub plugin):

• create Credential & Connection alias, fill in the Connection endpoint and its credentials;
• create REST message with variables and configure the headers and body that pulls in the variables;
• trigger the sending using Business Rule that in its course triggers a Flow (but could be enough only with the flow);
• created a flow action that received inputs from the flow, then uses a REST step that is populated with my Credential Alias and REST message layout.