How to copy the phone number of the sender of a text message?

I'm using a service to report SMS spam via text messages. First step is to forward the text message, and the second step is to send a text message containing the phone number of the spammer. I achieve the first step by doing a long press on the message itself, then I have a forward menu. But I can't find a reasonable way to do the second step. I can see the phone number in various places, but never in a selectable text field. So I have to create a new contact for the spammer, give him a name, tap the phone number text field, copy, and go back a few steps to create a new text message, then paste the previously copied phone number. Is there a faster way to do this ?