אורח חיים קכ״בOrach Chayim 122

Se’if 1א׳

אִם בָּא לְהַפְסִיק וְלַעֲנוֹת קַדִּישׁ וּקְדֻשָּׁה בֵּין י"ח לְיִהְיוּ לְרָצוֹן, אֵינוֹ פּוֹסֵק, שֶׁיִּהְיוּ לְרָצוֹן מִכְּלַל הַתְּפִלָּה הוּא; אֲבָל בֵּין יִהְיוּ לְרָצוֹן לִשְׁאָר תַּחֲנוּנִים, שַׁפִּיר דָּמִי. הַגָּה: וְדַוְקָא בְּמָקוֹם שֶׁנּוֹהֲגִין לוֹמַר יִהְיוּ לְרָצוֹן מִיָּד אַחַר הַתְּפִלָּה, אֲבָל בְּמָקוֹם שֶׁנּוֹהֲגִין לוֹמַר תַּחֲנוּנִים קֹדֶם יִהְיוּ לְרָצוֹן, מַפְסִיק גַּם כֵּן לְקַדִּישׁ וּקְדֻשָּׁה; וּבִמְקוֹמוֹת אֵלּוּ נוֹהֲגִים לְהַפְסִיק בֶּאֱלֹהַי נְצֹר, קֹדֶם יִהְיוּ לְרָצוֹן, לָכֵן מַפְסִיקִין גַּם כֵּן לִקְדֻשָּׁה וּלְקַדִּישׁ וּלְבָרְכוּ (דִּבְרֵי עַצְמוֹ לְפי' רַשְׁבָּ"א שֶׁהֵבִיא הַבֵּית יוֹסֵף); וּמִיהוּ הָרָגִיל לוֹמַר תַּחֲנוּנִים אַחַר תְּפִלָּתוֹ, אִם הִתְחִיל שְׁלִיחַ צִבּוּר לְסַדֵּר תְּפִלָּתוֹ וְהִגִּיעַ לְקַדִּישׁ אוֹ לִקְדֻשָּׁה, מְקַצֵּר וְעוֹלֶה, וְאִם לֹא קִצֵּר יָכוֹל לְהַפְסִיק כְּדֶרֶךְ שֶׁמַּפְסִיק בַּבְּרָכָה שֶׁל קְרִיאַת שְׁמַע, אֲפִלּוּ בָּאֶמְצַע.

Laws that are Applicable Between Sh'moneh Esrei and "Yih'yu L'Ratzon". Containing 3 S'ifim: If one is inclined to interrupt [one's prayer] to respond to Kaddish or K'dusha between [the end of] Sh'moneh Esrei and "Yih'yu L'Ratzon" ["May it be acceptable"], one does not interrupt; for "Yih'yu L'Ratzon" is included in the [Sh'moneh Esrei] prayer. But between "Yih'yu L'Ratzon" and the rest of the supplications [that are said afterwards], it is fine [to interrupt]. Gloss: And this is specifically in a place where it is practiced to say "Yih'yu L'Ratzon" immediately after the [Sh'moneh Esrei] prayer. But in a place where they practice by saying supplications before "Yih'yu L'Ratzon", one may interrupt also for Kaddish and K'dusha. And in these places, it is practiced to interrupt in "Elokai, Netzor" ["My God, guard"], before "Yih'yu L'Ratzon". And therefore, we interrupt also for K'dusha, Kaddish, and Bar'khu. [His own words, following to the Rashba that was brought by the Beit Yosef]One who was accustomed to say supplications after his [Sh'moneh Esrei] prayer - if the prayer leader began to order [i.e. recite] his [repetition of the] prayer and reached Kaddish or K'dusha, one should truncate [one's supplications] and stand up. And if one did not truncate [one's supplications], one may interrupt in the same way that one interrupts in a blessing of the Recitation of the Sh'ma; even in the middle of [one's recitation].

Se’if 2ב׳

אֵין נָכוֹן לוֹמַר תַּחֲנוּנִים קֹדֶם יִהְיוּ לְרָצוֹן, אֶלָּא אַחַר סִיּוּם י"ח מִיָּד, יֹאמַר: יִהְיוּ לְרָצוֹן; וְאִם בָּא לַחֲזֹר וּלְאָמְרוֹ פַּעַם אַחֶרֶת אַחַר הַתַּחֲנוּנִים, הָרְשׁוּת בְּיָדוֹ.

It is not proper to say supplications before "Yih'yu L'Ratzon", rather, after the completion of the Shemoneh Esrei, one immediately says "Yih'yu L'Ratzon", and if one wants to go back and say it another time after the supplications, the permission [to do so] is in one's hands.

Se’if 3ג׳

הָרָגִיל לוֹמַר ד' דְּבָרִים אֵלּוּ, זוֹכֶה וּמְקַבֵּל פְּנֵי שְׁכִינָה: עֲשֵׂה לְמַעַן שְׁמָךְ, עֲשֵׂה לְמַעַן יְמִינָךְ, עֲשֵׂה לְמַעַן תּוֹרָתָךְ, עֲשֵׂה לְמַעַן קְדֻשָּׁתָךְ.

One who is accustomed to say these 4 things will merit to greet "the face" of the Shechina: "Act for the sake of Your Name. Act for the sake of Your right hand. Act for the sake of Your Torah. Act for the sake of Your holiness."