The Prophet ﷺ taught a du'a for stepping into the masjid and another for stepping out: mercy on the way in, bounty on the way out. The minutes between the adhan and the iqamah are a time when du'a is not turned away.
Ad-du'a'u la yuraddu bayna-l-adhani wa-l-iqamah.
“The supplication made between the adhan and the iqamah is not rejected.”
Not a fixed wording but an open door. In the quiet minutes before the iqamah, ask Allah (SWT) for whatever you need, in any words.
Allahumma-ftah li abwaba rahmatik.
“O Allah, open for me the doors of Your mercy.”
The Prophet ﷺ said that when one of you enters the masjid, let him say this (Sahih Muslim 713). In Abu Dawud's narration (465) he first sends salam upon the Prophet ﷺ.
Allahumma inni as'aluka min fadlik.
“O Allah, I ask You of Your bounty.”
Taught in the same hadith: mercy is asked for on the way in, bounty on the way out (Sahih Muslim 713).