How do I enter an underscore in a text on my Samsung cell phone?

Use the hackers keyboard it is a

pc layout keyboard, there are cntrl alt tab arrows everything is present in it.

Anyways double click on 123 you will see dash changed into underscore.

The place of dash on first click on 123 button is of underscore if you click 123 one more time.

One first layout of symbols

Two second out of another pair of symbol

Three return to qwert.

When you have the text keyboard, hit the NUMBER button to bring up and numbers and other symbols and the underscore should be one of them.

And depending on which keyboard you are using it, it may be one of the letters or numbers on the main keyboard that needs a SHIFT to access the underscore.