What's the equivalent of PostMethod$addParameter() in http client 4.25

What's the equivalent of PostMethod$addParameter() in http client 4.25

i am moving some legacy code from apache commons http client (3.0.1) to
http client 4.25, and no longer see the PostMethod$addParameter() methods
for passing [name,value] pairs to the server....
it appears with http client 4.25 that i have to create some sort of
HttpEntity, which i can then bind to my request using
HttpPost$setEntity()....
in my case, i have a set of parameters whose values are (long!!) Base64
encoded text strings; so i don't want to send these parameters via the
URL, but rather place them in the body of the request...
what's the basic idiom for doing this with http client 4.25???