The link was attached to a statement from Strzok's lawyer, who wrote that the firing was 'deeply troubling to all Americans'.
The FBI did not have immediate comment.
The president also said his firing should mean that the probe into Hillary Clinton's email server - which Strzok oversaw - should be reopened.
Both Mr Comey and Mr McCabe said, however, they believed they were fired because they are key witnesses in special counsel Robert Mueller's probe into whether Mr Trump might have tried to obstruct the investigation.
The fired Federal Bureau of Investigation agent has launched a GoFundMe campaign aimed at raising $150,000 to pay for "hefty" legal costs and lost income. "This investigation is not politically motivated; it is not a witch hunt; it is not a hoax", he said. "No Collusion, No Obstruction - I just fight back!"
He also called for the investigation into Clinton's emails to be "redone".
Strzok and Page's story came to light just one day after former national security adviser Mike Flynn concluded his guilty plea in the Russian Federation case with Justice Department special counsel Robert Mueller in December 2017. "The firing of anti-Trump FBI agent Peter Strzok is welcome, if overdue". The next day, Mueller expelled Strzok from the group.
The lawyer with whom Strzok was texting, Lisa Page, had been part of the special counsel investigation, but no longer works for the FBI.
Within hours of the news, Strzok fired off his first tweet, which included a link to his new gofundme page.
Democrats accused Republicans of trying to divert attention from Mueller's investigation and Trump's ties to Russian Federation by focusing on Strzok.
For months, the pair's behavior has been the subject of many (bizarre) Trump tweets, cited as evidence that the supposed "witch hunt" investigation into Trump was biased from the start. The others are ex-FBI Director James Comey and FBI deputy director Andrew McCabe.
FBI Inspector General Michael Horowitz wrote after probing Strzok and Page that Strzok showed a "willingness to take official action" to hurt Trump, citing a text he sent telling Page "we'll stop" Trump from getting elected.
But he had remained an employee, including apparently following the FBI's decision to have him escorted out of its Washington, D.C., headquarters in June.
Goelman said the head of the office that normally handles disciplinary actions determined Strzok should be demoted and suspended for 60 days.