Siman 424
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א מַזְכִּירִין יַעֲלֶה וְיָבֹא בְּבִרְכַּת הַמָּזוֹן, וְאִם לֹא אָמַר אֵין מַחֲזִירִין אוֹתוֹ. וְאִם נִזְכַּר קֹדֶם שֶׁהִתְחִיל הַטּוֹב וְהַמֵּטִיב, אוֹמֵר: בָּרוּךְ שֶׁנָּתַן רָאשֵׁי חֳדָשִׁים לְעַמּוֹ יִשְׂרָאֵל לְזִכָּרוֹן. הַגָּה: וְעַיֵּן לְעֵיל סִימָן קפ"ח סָעִיף ז'.
1 The law of bringing up "Ya'aleh v-Yavo" in the grace after meals, which has one clause: We bring up "Ya'aleh v-Yavo in the grace after meals. And if it was not said, it is not necessary to repeat it. And if it is remembered before one started the fourth blessing, say, "Blessed be the one who gave Rosh Ḥodesh to His people Israel as a reminder." Rema: See above, SA Orach Chayim 188:7.