How to reset admin password on an external boot drive (running High Sierra)?

My eventual goal is to use an iMac running High Sierra to make a USB installer of Mojave for a 2010 Intel MacPro (I can’t be done from my Mac Studio M2 Ultra running Sequoia). But I can’t get anywhere because suddenly the iMac is not recognizing my Admin password. The standard methods of resetting an admin password (e.g., “resetpassword”) won’t work because the iMac boots only from an external SSD, which is not seen in any of the Recovery modes (e.g., Cmd+R).

How can I reset the admin password on that external boot drive? The iMac’s internal HD has only data on it, no bootable OS, so there’s no other boot drive. Is there any way to run “resetpassword” on an external drive? Thanks in advance. – K