If you need to copy part of an SMS message and paste it into another app, the easiest way is to use the “Forward” function in BIG SMS.
Open the message with Forward
Open BIG SMS and choose the message you want to copy. Tap the message to show its details, then tap “Forward”. The SMS editor opens with the selected message already inserted into the text field.
Copy the text using Android selection tools
Now you can use the standard Android text selection tools. Touch and hold the text, select the part you need, or select the whole message, and choose “Copy”. The text is now copied to the clipboard and you can paste it into another app.