How to concatenate base64 encode result?

[#1]

I have a stream in which I generate a json file with GenerateFlowFile, then I use Base64EncodeContent to encode the json file. I need to take the base64 encode result and concatenate it with a sentence, to finally move to invokehttp. I tried MergeContent for this and was not successful.
I'm not able to find a way to get the result of the Base64EncodeContent encode and concatenate it with the sentence. I would like suggestions on how to carry out this flow.
Below is the flow I drew:

[#2]

How did you eventually solve this?

[#3]

The ReplaceText procesor has a prepend/append mode which might be of help

[#4]

I couldn't find a solution for this.