You need to set your permalink structure to allow for the /mycustompost/admin or rather
/myposttype/mypostname
https://codex.wordpress.org/Class_Reference/WP_Rewrite
Might be of assistance
You need to set your permalink structure to allow for the /mycustompost/admin or rather
/myposttype/mypostname
https://codex.wordpress.org/Class_Reference/WP_Rewrite
Might be of assistance