FileMaker is not Excel. This is not a built in capability of FileMaker. Best you can do is add a Global field to the header, paste into it and then have a scritp (onObjectModify can perform it), parse the pasted data into a range of selected records (or create a set of new records using the pasted data...)
Steve, do you (perhaps) mean that you want to enter the same "text" into multiple records, into the same field? This can be done using the Replace Field Contents. It is a very useful tool; it is also a dangerous tool, if you are not (very) careful. It can quickly add (or change) the active field (which you clicked into), in all records of the found set. It is critical that you have ONLY the records you want to change (that field) as the Found Set.
If you only want to change the field in SOME of the records, while keeping the existing Found Set (also), then you could use the New Window (script step; visible in the Windows menu), then get just the ones you want to change at the found set there.
Once you have the records you want, you can click into the field; enter the data you want them all to get. Then run the Replace Field Contents (in Records menu, near the bottom). The default option will show the data (near the top). You can then click "Replace" (at the bottom). [ Notice it is at the LEFT of default button, "Cancel", so you must click "Replace" (or nothing will happen). This is a safety feature.]
You cannot (as far as I know) use Edit, "Undo" the above action [ I think because it changes multiple records]. If there is nothing in the field yet, it is not so critical. If not, it is common to somehow "save" that data before doing this. If a local file, you can save the file. If not local, you could do a Replace Field Contents, of what exists in the field, into a "backup" field (so you at least have the data if things go wrong; like if you're doing a calculation of somewhat questionable result).