The simple answer is tilde "~", which you already know.  (If you didn't, now you do)

But given a phrase, should all the words be in the same line?  Should there be a line break?  What happen when you have many words, like "I have a dream"?

By default, Tagxedo applies some black magic to figure out how to break a phrase into multiple lines -- it prefer tags that are neither too thin nor too fat.  Tags that are "just right" are both aesthetically more pleasing, and easier to fit into shapes.

That said, you can force Tagxedo to break the line in a certain way.  By default (single tilde), Tagxedo makes the decision.  Two tildes ("~~") means the two neighboring words must be on the same line, and three tildes "~~~" means the two neighboring words must be on different lines.  Here's the example (Combine Identical Words = No, Remove Common Words = No):

A~~B~~C~~D~~E~~F~~G~~~H~~I~~J~~K~~L~~M~~N~~O~~P~~~Q~~R~~S~~~T~~U~~V~~~W~~X~~Y~~and~~Z

And the result:

This enhancement is quite recent.  Oh, and the words are now centered (instead of left-aligned).