Basic prompt:
1boy/1girl, [type of tentacles] tentaclejob

Example:
1boy, gold tentaclejob

You may get better results using one or more of the following tags:
pubic hair, erection, tentaclejob milking, penis peek

Note:
When using the tag tentaclejob milking you may want to add milking machine and cum in the negative prompt.

Negative prompt:
oral, internal view, tentacles, cum

Avoid:
penis

Note:
Using the tag penis tends to generate a tentacle inside the character’s mouth.