What is the rule to use a myślnik between a word and its ending when declined?
You simply use it before the endings added to acronyms:
(Nom) SMS, (Gen) SMS-a, (Dat) SMS-owi, (Acc) SMS or SMS-a, (Inst) SMS-ie, (Loc) SMS-em. Also in plural (SMS-y, SMS-ów and so on).
If you choose the other spelling (esemes), which is correct according to dictionaries, but (for now) less common, you don't need the "myślnik" (esemes, esemesa, esemesowi, esemes or esemesa, esemesie, esemesem, Plural esemesy, esemesów etc.), because in Polish you don't use this graphic sign (-) in normal declension.
On a side note, there are two forms (in singular) possible:
Wysłać SMS
Wysłać SMS-a
the first one (SMS) is "more correct" according to the historical declension rules for non-animate male nouns (*), but the second one (SMS-a) is stronger in the modern language, where newly introduced male nouns (SMS or esemes, e-mail or mail) and some a little older (gem, set, but not mecz!) assume the declension of the animate male nouns, and things get pretty ugly here, because they also change the grammatical form required for an adjective! (**)
(*) Accusative case (biernik) of male nouns is typically identical
1/ with the Nominative case (mianownik), so no additional ending, for
non-animate nouns, for example:
widzę/słyszę/kupuję/wysyłam (and many, many other verbs) stół, komputer, telefon, tapczan, list, żyrandol, dywan etc.
2/ with the Genitive case (dopełniacz), so usually -a ending, for
animate nouns, for example:
widzę/słyszę/kupuję/odwiedzam ... ojca, syna, brata, studenta, psychologa, lekarza, psa, konia, kota, ślimaka, pierwotniaka, wirusa etc.
(**) Examples with adjectives
Accusative male noun + adjective
(animate nouns, "młody pies" = "young dog")
Widzę młod
ego psa.
(non-animate nouns, "nowy dom" = "new house")
Widzę now
y dom.
If you decide to use the historically correct declension:
Wysyłam długi e-mail, SMS, list.
Venus Williams przegrała pierwszy set 5-7, po czym wygrała morderczy tie-break (18-16) w drugim secie.
If you choose the declension prevailing in the modern language:
Wysyłam długi
ego e-mail
a, SMS
-a (you can't use it with "list").
Venus Williams przegrała pierwsz
ego set
a 5-7, po czym wygrała mordercz
ego tie-break
a (18-16) w drugim secie.