Problem with Authorization Header and Base64 Encoding in HTTP Request Activity

I am trying to make an API(Type- Basic Auth, also expect header as "INSTCTID") call from the HTTP Request activity in Elsa Studio.

{
"INSTCTID": "123456",
"Authorization": "Basic {{ 'username:password' | base64 }}"
}

I receive the following error:
status code: 401
"{"Cache-Control":["public, no-cache"],"Pragma":["no-cache"],"X-Frame-Options":["Deny"],"X-XSS-Protection":["1; mode=block"],"X-Content-Type-Options":["nosniff"],"X-Permitted-Cross-Domain-Policies":["none"],"Referrer-Policy":["origin"],"Strict-Transport-Security":["max-age=31536000; includeSubDomains"],"Date":["Mon, 03 Mar 2025 06:14:09 GMT"]}"