If your airgun is made before November 2020 the Texan .357 adapter #A34 will fit the barrel, but will need to be bored out to accommodate the .45 clearance.
When you order the adapter add a note on checkout to bore it out to a .49 bore diameter to clear the adapter.
If your airgun is made after november 2020 the barrel has a 5/8x24 thread, The adapter from BIG Bore converts the thread to M18x1.
I recommend the Emperor .45 M18x1 #S5B.