Hi Ricardo
To do this dynamically you'll have to build the Authorization header yourself. The correct format is:
"Basic " + EncodeToBase64(username:password)
In a REST API, add an input parameter with the name Authorization that is sent in header:

Then add the reference to the methods BinaryToBase64 and TextToBinaryData from the BinaryData extension, and assign the following value to the input parameter:

Let me know if it works for you